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KTH Royal Institute of Technology

Screen reader users will see links on a page listed without context, 
so you should ensure the same link text is not used to point to different web
addresses.

For example:

```html
<p>
    Learn the latest about our products: 
    <a href="/products/news">News</a>
</p>

<p>
    Learn the latest about our services: 
    <a href="/company/news">News</a>
</p>
``` 

To a screen reader who is listing links, they will only hear:

* News
* News

These links appear to be the same, but they both behave differently. 
One solution, which doesn't require the page design to change, is to add an 
`aria-label` for each link:

```html
<p>
    Learn the latest about our products: 
    <a href="/products/news" aria-label="Product news">News</a>
</p>

<p>
    Learn the latest about our services: 
    <a href="/company/news" aria-label="Company news">News</a>
</p>
``` 

Which will be heard as:

* Product news
* Company news

To pass this checkpoint, make sure that links which point to different pages 
have distinct text inside them, or that they specify ARIA labels (`aria-labelledby` or
`aria-label`), or they use hidden text only visible to screen readers
(e.g. `<span class="sr-only">...</span>`) to distinguish them from each 
other.

If links point to the same page (e.g., they redirect or use tracking
parameters, but are functionally equivalent to the user) you can ignore
this issue.

a-duplicate-link-text-for-different-destinations

To comply with the strictest accessibility standard (WCAG AAA), the colour of text must 
strongly contrast with its background color, so that people with significant visual impairments 
can read it.

The [contrast ratio](https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html#contrast-ratiodef)
must be at least 7:1 for body text, and 4.5:1 for large text.

A far less demanding version of this check (1.4.3) exists for WCAG AA. All websites should aim to 
meet the easier AA standard at a minimum. The AAA standard is desirable but far more challenging,
and may not be practical for your website.

aaa-text-contrast

Links must be defined in a specific way to be accessed by screen readers, which are used by 
blind and the partially-sighted.

To work with accessibility technologies, links must contain both machine-readable text, and 
a `href` attribute. If a link is not actually
pointing to a page or part of a page, it should be replaced with another semantic element, such
as a `button`, or the `<a>` tag should have a role of `button`. 

For example:

```html
<a href="/about">About us</a>
```

If the link contains _nothing but an image_, that image should specify alternative text, e.g.

```html
<a href="/apple">
    <img src="apple.png" alt="Apple">
</a>
```

Before HTML5, an anchor was often used to create a target to link to, e.g.

```html
<a name="example"></a>
```

This is not supported in HTML5, and the use of anchors for targets is discouraged. Use an `id`
attribute on a non-anchor tag instead, e.g.

```html
<div id="example"> ... </div>
```

Empty links (e.g. `<a></a>`) are often incorrectly used for buttons or controls in a
JavaScript application. In these cases, you should usually define the link as having a more appropriate
role, e.g. as a button. To do this, use the `role` attribute (e.g `<a role="button">`), or replace 
the `<a>` tag with a more appropriate tag, such as `<button>`.

For more details, see [Technique H91](https://www.w3.org/TR/2016/NOTE-WCAG20-TECHS-20161007/H91)
and learn about [ARIA roles](https://www.sitepoint.com/how-to-use-aria-effectively-with-html5/).

bad-anchor

All `id` attributes on a page must be unique, or accessible technologies used by the page 
can stop working.

## Example

The duplicate `id` here makes it unclear which label belongs to which input field:

```html
<label for="name">Your name</label>
<input id="name">

<label for="name">Company name</label>
<input id="name">
```

As a result, accessible technologies like screen readers can fail to read out the 
correct text, or may read out no text at all.

Similar issues can occur when using `aria-labelledby` and `aria-describedby`.

## How to fix

Ensure all `id` attributes on a page are unique. 

## More information

See [Technique H93](https://www.w3.org/TR/2016/NOTE-WCAG20-TECHS-20161007/H93) 
for more information.

duplicate-id

When selecting a user interface component with the keyboard, the focus indicator must 
not be _partly_ obscured by anything in front of it. 

## Why this matters

When using a keyboard, it is important that the user can see where they are on the page.
If the focus indicator is obscured, the user may not know where they are, and may not
be able to interact with the page.

## Examples

Most common examples include:

* Banners at the top or bottom of the page, such as cookie banners
* Fixed navigation bars
* Fixed footers
* Popup modals, such as cookie consent modals

## Testing this manually

* Open the page
* Press the tab key to move focus through elements on the page
* Check that the focus indicator is not fully obscured by anything in front of it

## Comparison with Level AA version

This is the stricter (Level AAA) version of the Level AA check (2.4.11).

The Level AA version only flags issues where the focus indicator is _completely_ obscured, this
version flags issues where the focus indicator is _partly_ obscured.

## How to fix

With elements that cover part of the screen, such as cookie banners, ensure
that the user can scroll to see any content behind the banner, without having to close
the banner.

The two most very common solutions are:

1. Avoid the use of fixed elements that could obscure other elements
2. Use the CSS `scroll-margin` property (see [WCAG Technique C43](https://www.w3.org/WAI/WCAG22/Techniques/css/C43))

With popup modals, it should usually be impossible to focus anything behind the modal.
Most common modal frameworks will automatically trap focus within the modal, so that
this is impossible. Example: [Bootstrap Modal](https://getbootstrap.com/docs/5.2/components/modal/#live-demo).

## Methodology

1. Silktide simulates a keyboard user tabbing through the page
2. Silktide simulates the focused element being vertically scrolled into the center of the page
   (see 'Rationale for scrolling' below)
3. For each element that receives focus, Silktide checks that the focus indicator is not obscured
   by another element on top of it
4. If the focus indicator is obscured, Silktide raises an issue

### Rationale for scrolling

The rationale for point #2 (above) is worth expanding. At different screen sizes, the amount of scrolling
a browser makes between tabs will vary. For example, Chrome will typically avoid scrolling until a
focus element falls outside the current viewport, and then it will adopt the least amount of
scrolling possible to bring the element into view.

This means if you test a page in Silktide, and in a browser with a fractionally different height,
you may experience different elements being obscured or not, especially elements blocked by sticky headers
and footers. Every only a few pixels of difference in screen height can make a huge difference the
further down the page you go.

In all these cases, the use of user-directed vertical scrolling, e.g. via the cursor keys, would permit
the focused element to be seen. Whether that would be necessary would depend on the exact browser and
size of the browser viewport.

Silktide purposely adopts a generous frame here, to avoid flagging issues which might only affect the
exact screen size that it tested. In our testing, when we didn't do this, the number of subjectively false
positives could be very high, and extremely confusing, as they would not replicate during manual testing on
anything but the exact same browser size.

Essentially, Silktide chooses not to flag issues which _could_ affect some very particular screen sizes
_inconsistently_, but it will flag issues that should affect _all_ screen sizes consistently.

The W3C does not specify how to test this Success Criterion.

### Simulating conditional modals

Silktide adopts additional behavior to simulate the use of dialog modals that may be opened by user
behavior after a page loads (i.e. modals that don't automatically open when a page loads).

If a non-opened modal is detected, Silktide assumes that _elements outside the modal cannot be focused_.
This is not guaranteed, but it is impossible for Silktide to predict the behavior of a modal that has not
yet been opened, and this is much more accurate than assuming that the modal does not constrain focus.

As a result, a modal which is incorrectly programmed (i.e. doesn't constrain focus) may not be detected
as obscuring focus behind it, if the modal is not opened by default.

## Further reading

* [WCAG 2.4.12](https://www.w3.org/WAI/WCAG22/Understanding/focus-not-obscured-enhanced)
* [WCAG Technique C43 - Using CSS margin and scroll-margin to un-obscure content](https://www.w3.org/WAI/WCAG22/Techniques/css/C43)

ensure-focus-is-not-obscured-enhanced

When selecting a user interface component with the keyboard, the focus indicator must 
not be _completely obscured_ by anything in front of it.

## Why this matters

When using a keyboard, it is important that the user can see where they are on the page.
If the focus indicator is obscured, the user may not know where they are, and may not
be able to interact with the page.

## Examples

Most common examples include:

* Banners at the top or bottom of the page, such as cookie banners
* Fixed navigation bars
* Fixed footers
* Popup modals, such as cookie consent modals

## Testing this manually

* Open the page
* Press the tab key to move focus through elements on the page
* Check that the focus indicator is not fully obscured by anything in front of it

## Comparison with Level AAA version

This is the less strict (Level AA) version of the Level AAA check (2.4.12).

This Level AA version only flags issues where the focus indicator is _completely_ obscured, the
Level AAA version flags issues where the focus indicator is _partly_ obscured.

## How to fix

With elements that cover part of the screen, such as cookie banners, ensure
that the user can scroll to see any content behind the banner, without having to close
the banner.

The two most very common solutions are:

1. Avoid the use of fixed elements that could obscure other elements
2. Use the CSS `scroll-margin` property (see [WCAG Technique C43](https://www.w3.org/WAI/WCAG22/Techniques/css/C43)) 

With popup modals, it should usually be impossible to focus anything behind the modal.
Most common modal frameworks will automatically trap focus within the modal, so that
this is impossible. Example: [Bootstrap Modal](https://getbootstrap.com/docs/5.2/components/modal/#live-demo).

## Methodology

1. Silktide simulates a keyboard user tabbing through the page
2. Silktide simulates the focused element being vertically scrolled into the center of the page
   (see 'Rationale for scrolling' below)
3. For each element that receives focus, Silktide checks that the focus indicator is not obscured
   by another element on top of it
4. If the focus indicator is obscured, Silktide raises an issue

### Rationale for scrolling

The rationale for point #2 (above) is worth expanding. At different screen sizes, the amount of scrolling 
a browser makes between tabs will vary. For example, Chrome will typically avoid scrolling until a
focus element falls outside the current viewport, and then it will adopt the least amount of 
scrolling possible to bring the element into view.

This means if you test a page in Silktide, and in a browser with a fractionally different height, 
you may experience different elements being obscured or not, especially elements blocked by sticky headers 
and footers. Every only a few pixels of difference in screen height can make a huge difference the 
further down the page you go.

In all these cases, the use of user-directed vertical scrolling, e.g. via the cursor keys, would permit 
the focused element to be seen. Whether that would be necessary would depend on the exact browser and
size of the browser viewport.

Silktide purposely adopts a generous frame here, to avoid flagging issues which might only affect the 
exact screen size that it tested. In our testing, when we didn't do this, the number of subjectively false
positives could be very high, and extremely confusing, as they would not replicate during manual testing on
anything but the exact same browser size.

Essentially, Silktide chooses not to flag issues which _could_ affect some very particular screen sizes 
_inconsistently_, but it will flag issues that should affect _all_ screen sizes consistently.

The W3C does not specify how to test this Success Criterion.

### Simulating conditional modals

Silktide adopts additional behavior to simulate the use of dialog modals that may be opened by user
behavior after a page loads (i.e. modals that don't automatically open when a page loads).

If a non-opened modal is detected, Silktide assumes that _elements outside the modal cannot be focused_.
This is not guaranteed, but it is impossible for Silktide to predict the behavior of a modal that has not
yet been opened, and this is much more accurate than assuming that the modal does not constrain focus.

As a result, a modal which is incorrectly programmed (i.e. doesn't constrain focus) may not be detected 
as obscuring focus behind it, if the modal is not opened by default.

## Further reading

* [WCAG 2.4.11](https://www.w3.org/WAI/WCAG22/Understanding/focus-not-obscured-minimum)
* [WCAG Technique C43 - Using CSS margin and scroll-margin to un-obscure content](https://www.w3.org/WAI/WCAG22/Techniques/css/C43)

ensure-focus-is-not-obscured-minimum

All interactive components on a page, such as buttons or menus, should be far enough apart 
from other interactive areas, to avoid them being used by mistake. 

This ensures people with motor impairments can easily click or tap on the right element.

## How to fix

* Ensure all interactive elements are at least 24 CSS pixels wide and tall
* OR ensure there is at least 24 CSS pixels (minus the width and height of the element)
  between each interactive element

## Methodology

This check is run on both the mobile and desktop versions of every page. Results
for each may differ.

1. All interactive elements are identified, including buttons, links, and form controls.
2. If an element is not normally visible, it is ignored. This includes elements which
   are positioned offscreen or cropped (e.g. for 'screen-reader only' text).
3. If an element is larger than 24 CSS pixels in both dimensions, it passes.
4. Otherwise, the element is compared to all other elements on the page.
5. The minimum horizontal gap between the element is calculated as 24 CSS pixels,  
   minus the width of the element.
6. The minimum vertical gap between the element is calculated as 24 CSS pixels,  
   minus the height of the element.
7. If another element falls within the minimum horizontal or vertical gap, the 
   element fails, unless both are links that point to the same place.

## Further reading

* [Target Size (Minimum)](https://www.w3.org/WAI/WCAG22/Understanding/target-size-minimum.html)
* [Technique C42 - Using min-height and min-width to ensure sufficient target spacing](https://www.w3.org/WAI/WCAG22/Techniques/css/C42)

ensure-minimum-target-size

Most content should be written so people with a lower secondary level of education can understand it.
Where this is not possible, supplemental content or an alternative version should be provided.

This check highlights pages with a reading age of 16 years or higher. Only pages with at least 100 words
of content (excluding navigation, footers, etc) are considered.  

For more information, see [WCAG SC 3.1.5](https://www.w3.org/TR/UNDERSTANDING-WCAG20/meaning-supplements.html).

ensure-reading-ability-is-not-advanced

All interactive components on a page, such as buttons or fields, should ideally be large enough 
to be easily used by people with motor impairments. 

This ensures people with motor impairments can easily click or tap on the element.

The recommended size is 44 x 44 CSS pixels, with some exceptions:

* **Equivalent available** - the functionality provided by
  the control is available elsewhere, and large enough, on the same page.
* **Inline text** - the control appears inside a sentence or block of text. 
* **User Agent control** - the size of the target is determined by the user agent 
  and is not modified by the author. Generally this is not the case for web content.
* **Essential** - the size of the target is essential to the information being conveyed,
  e.g. a tutorial demonstrating what a small button looks like. This exception is rare.

## Applying this check

Nearly all websites will fail this check by default. The minimum size requirement
is larger than most links and controls on most websites.

This is the much stricter version of the WCAG 2.2 AA check for target size. Most websites
should focus on the AA check before considering this AAA check:

* [Target Size (Minimum)](https://w3c.github.io/wcag/understanding/target-size-minimum.html)

## Methodology

This check is run on both the mobile and desktop versions of every page. Results
for each may differ.

1. All interactive elements are identified, including buttons, links, and form controls. 
2. If an element is preceded or followed directly by a non-empty text node, the control
   is ignored.
3. If an element is not normally visible, it is ignored. This includes elements which 
   are positioned offscreen or cropped (e.g. for 'screen-reader only' text).
4. If an element is a link, and another visible link on the page points to the same URL, with 
   that other link meeting the size requirements, the element is ignored.
5. Otherwise, the element must have a minimum size of 44 x 44 CSS pixels.

## Troubleshooting

Silktide only records the first 100 instances of a failure on a page. Because failures
of this check are common, it is possible that your page has more than 100 failures, but
only the first 100 will be shown.

## Further reading

* [Target Size (Enhanced)](https://www.w3.org/WAI/WCAG22/Understanding/target-size-enhanced)

ensure-reasonable-target-size

All frames must define a readable title, which briefly described the contents of that frame.

Note that this is not the same as a `<title>` tag inside the iframe, which is insufficient.
You must provide a meaningful description for the frame using the `title` attribute.

Examples:

```html
<iframe title="Advertisement" src="ad.html">
```

```html
<frame title="Advertisement" src="ad.html">
```

# Why this matters

Accessible technologies such as screen readers rely on the `title` attribute to list
the frames on a given page. 

Without a title, it is impossible to know what the frame contains, and the user must
navigate inside it, and explore the contents of the frame to figure this out.

Often iframes are used for technologies like banner ads, videos, or chat widgets. The 
absence of a `title` makes these areas of the page much harder to identify, understand, 
or use.

# How titles are tested

Frames which are not presented to screen readers are ignored. This includes invisible
frames, and frames marked with `aria-hidden`.

A `title` attribute must contain meaningful text, e.g. numbers / letters, to be considered
valid. A title containing just spaces, or non-readable text (e.g. an emoji) will
be flagged as invalid.

# More information

Learn about [Technique H64](https://www.w3.org/WAI/WCAG21/Techniques/html/H64)
as part of [SC 2.4.1 - Bypass Blocks](https://www.w3.org/WAI/WCAG21/Understanding/bypass-blocks).

has-frame-title

The purpose of a link should be clear from the text inside the link. Links like "learn more" are
not helpful to users with assistive technology.

Links should also avoid redundantly saying that they are links, e.g. "link to page" as screen
readers will read these out with the word "link" added, e.g. "link link to page".

Exceptions to these rules are allowed where the purpose of a link would be ambiguous to users 
in general.

link-purpose-a

Avoid opening links in a new tab or window. Where you must do this, include an explanation
in the text of the link.

## Why this matters

Opening links in a new tab or window breaks the back button and can be disorientating for 
novice web users, and the visually impaired. Generally you should avoid opening links in a
new tab or window, but if you have to, you should include text explaining that this will happen.

## How to fix

An explanation must be clear in the accessible text for the link. For example:

```html
<a href="products.html" target="_blank">
    Products (opens in new window)
</a>
```

Or, for an image inside a link:

```html
<a href="products.html" target="_blank">
    <img src="products.jpg" alt="Products (opens in new window)">
</a>
```

Rather than updating all of your links manually to do this, you can add a simple CSS
selector, such as:

```css
a[target="_blank"]:after {
    content: " (opens in new window)";
}
```

Note that doing this with an icon, is not sufficient to pass this check. The text must be
readable by screen readers.

Alternatively you can override the link text for accessible technologies (like screenreaders),
so that the appearance of the link does not change. For example:

```html
<a href="products.html" target="_blank" aria-label="Products (opens in new window)">
    Products
</a>
```

Note that if you do this, the `aria-label` will replace the text content of the link completely,
so it must still make sense in isolation.

## Terminology

Although it is common to say "opens in new window", almost all browsers now use tabs, and
so this terminology is potentially confusing. You may prefer: "opens in new tab".

## Methodology

1. Silktide considers all links on a page with a `target` attribute of `_blank`.
2. For each link, Silktide considers the screen-readable text of the link, which includes:
   * The text content of the link
   * Any `alt` text inside any images inside the link
   * Any `aria-label` or `aria-labelled-by` text
   * The `content` of the pseudo-elements `:before` and `:after`
3. This text is analysed for the presence of common text, including "opens new tab"
    and "opens new window". If the text is not found, the link fails the test.

## Further reading

Generally best practice is to avoid opening links in a new tab or window.  

The W3C provides detailed guidance on 
[when opening a link in a new window may be appropriate](https://www.w3.org/TR/WCAG20-TECHS/G200.html).

link-text-explains-open-in-new-window

Lists of related items should be written [semantically](https://html.com/semantic-markup/) as a 
list.

Usually this means rewriting these to use the `<ul>` or `<ol>` tags.

This allows assistive technologies to more easily navigate from item to
item, or past entire groups of links if the user wants to. 

## Examples 

### Navigation

This navigation:

```html
<a href="/about">About us</a>
<a href="/products">Products</a>
<a href="/contact">Contact us</a>
```

Should be rewritten as a list:

```html
<ul>
  <li><a href="/about">About us</a></li>
  <li><a href="/products">Products</a></li>
  <li><a href="/contact">Contact us</a></li>
</ul>
```

### Tables for content

This single-column table is redundant. There is no
value in placing the content in a table, which makes
it harder for screen reader users to interpret and interact
with.

```html
<table>
  <tr>
    <td>
      Banana
    </td>
  </tr>
  <tr>
    <td>
      Apple
    </td>
  </tr>
  <tr>
    <td>
      Pear
    </td>
  </tr>
</table>
```

Should be rewritten as a list:

```html
<ul>
  <li>Banana</li>
  <li>Apple</li>
  <li>Pear</li>
</ul>
```

## Justification

Users of assistive technologies, like screen readers, can skip an entire
list in one step. In some cases, they can even search inside a list. 

These aids are not available where links are written outside a list. 
Failing to use lists correctly can force users to have to step through
hundreds of items one at a time.

## Methodology

1. Any single-column tables are automatically considered 
   redundant lists. 
2. For non-table elements, recursively work down the page looking inside
   each element for links separated by minimal amounts of non-link text. 
   If the non-link text is below 30 characters 
   in length, we assume we're inside content that should be a list.
   Some exceptions are made, e.g. the presence of an `<hr>` would break
   out of the list.

## More information

See [Technique
H48](https://www.w3.org/TR/2016/NOTE-WCAG20-TECHS-20161007/H48) for
details.

mark-navigation-section-as-list

Images should specify appropriate
[alternative text](https://help.silktide.com/en/articles/8020957-alternative-text), 
where appropriate.

Alternative text is text that is used in place of an image if it cannot
be seen. This is important for users who are blind or partially sighted, 
who may have the page read to them by a screen-reader. Without alternative 
text, they will not know what the image is, or what it is for.

## How to fix this issue 

In most cases, you can fix this issue by adding alternative text to the media
in your Content Management System (CMS).

If you need to do this in code, you can alternative text to an image using 
the `alt` attribute. For example: 

```html
<img src="apple.png" alt="A red apple">
```

Alternative text should be descriptive and useful. We recommend you read 
[this comprehensive guide to writing alternative text](https://webaim.org/techniques/alttext/)
by WebAIM.

It is not always appropriate to provide alternative text. For example, if an
image is purely decorative - it adds nothing to the content of the page - then
it is better to omit the alternative text entirely. This is because screen-reader
users will have to listen to the alternative text, which can be annoying if it
is not useful, or highly repetitive.

To specify that an image is decorative, use an empty `alt` attribute, e.g.:

```html
<img src="decorative-image.png" alt="">
```

## Methodology

This check tests if the `alt` attribute is present on an image. If it is not, 
then the check fails.

Some images are excluded from this check:

1. **Images without a `src` attribute.** This is because they are
   not visible to users, and therefore do not need alternative text.
2. **Images which are 1px in width or height.** This is because they are only
   used for tracking pixels, or (rarely) layout purposes, and do not need 
   alternative text.
3. **Images which are hidden from screen readers using the `aria-hidden` attribute.**
   This is because they are not accessible to screen readers, and therefore would not 
   use their alternative text.

## More information

The W3C also provide more information on how to satisfy this check in 
[Technique H37](https://www.w3.org/TR/WCAG20-TECHS/H37).

missing-alt

Every page should have a clearly defined heading, known as a "H1" (Heading 1).

A `H1` is the main heading of the page. It helps accessibility by allowing uses to
skip to the main content of the page. It helps all users identify what the page is about. 
It helps SEO by giving special emphasis to the topic of the page.

It is not enough to simply include large text. Headings must be coded 
[semantically](https://html.com/semantic-markup/) as `<h1>` tags, like so:

```html
<h1>This is a heading</h1>
```

Note that headings must contain some machine readable text to be considered valid. 
For example, this heading is not valid:

```html
<h1>
    <img src="company-logo.png">
</h1>
``` 

But with the addition of `alt` text, this heading is valid:

```html
<h1>
    <img src="company-logo.png" alt="About Company Inc.">
</h1>
``` 

Generally we recommend you just use plain text for your headings.
See [Technique H42](https://www.w3.org/TR/WCAG20-TECHS/H42) for more details.

missing-h1

Form controls must appear sufficiently distinct from their surroundings, so that people 
with visual impairments are still able to clearly see them.    

Either the border or the form control itself must have a 3:1 colour contrast with the 
surrounding area. 

## Methodology

* All `<input>`, `<textarea>`, and `<select>` elements are checked.
* For `<input>` elements, these types are ignored as their
  appearance is controlled by the browser or operating system:
  `checkbox`, `radio`, `submit`, `reset`, `image`, `hidden`, `file`. 
* The flattened background color of the control is determined. This is the color of the
  control itself, combined with any parent elements if the control is transparent.
* If the control has a border or outline:
  * The flattened background color of each edge is determined. 
    This is the color of the edge itself, combined with any parent elements
    if the edge is transparent.
  * If any edge has sufficient contrast with the flattened background color of the control, 
    the control is considered to have sufficient contrast.
* If the control does not have a border or outline:
  * The flattened background color of the surrounding 
    area is determined. This is the color of the first control's parent element which is at least 
    one pixel larger around the element in every dimension, i.e. above, below, left, and right.
  * If the background color of the element has sufficient contrast with surrounding area color
    the control is considered to have sufficient contrast.

## Troubleshooting

There are some limitations to the contrast checker:

* The use of drop shadows, glowing effects, gradients, and other complex effects 
  mean there is no one clear color to determine the contrast ratio against. 
  In these cases, the contrast ratio cannot currently be determined.
* It is possible for an element to be positioned over multiple elements, e.g. via
  `position: absolute` or `position: fixed`. In this case, the contrast ratio cannot 
  currently be determined.

## Further reading

* [WCAG 1.4.11](https://www.w3.org/WAI/WCAG21/Understanding/non-text-contrast.html)

non-text-contrast

wcag-22.score

wcag-22-a.score

wcag-22-aa.score

wcag-22-aaa.score

website.pages

website.documents

When an image is the only content of a link, it should specify alternative text, 
via an `alt` attribute. The alternative text should describe the purpose of the link.

Without this, some users will be unable to determine what a link does, for example anyone 
using a screen reader.

To fix this, simply add an `alt` attribute to the image inside the link, e.g.:

```html
<a href="/apple">
    <img src="apple.png" alt="A red apple">
</a>
```

img-in-link-missing-alt

To comply with WCAG AA, the colour of text must sufficiently contrast with
its background color, so that people with moderate visual impairments can read it.

The [contrast ratio](https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html#contrast-ratiodef) 
must be at least 4.5:1 for body text, and 3:1 for large text.

A much more demanding version of this check (1.4.6) exists for WCAG AAA. Ignoring an issue in this check 
will also ignore it in that check, and vice versa.

aa-text-contrast

Tables which contain information should have clear headers defined.  

In this example, the top row of the table is a header:

| Surname        | City          |
| -------------- | ------------- |
| Washington     | London        |
| Phoenix        | Arizona       |
| Austin         | Travis        |

Notice how without the header here, it would be almost impossible to understand 
what the individual values mean ("Washington" could be a city or a surname). 

It is not enough to just make the headers look differently, as accessible technologies will not 
recognize this (for example, a screen reader cannot 'see' different colors or fonts). 
Table headings must be defined using the `<th>` tag. 

This example shows how a table should use a `<th>` to mark cells as headings. Note the 
use of the `col` attribute, to specify these headings are for the _column_:

```html
<table>
  <tr>
    <th scope="col">First name</th>
    <th scope="col">City</th>
  </tr>
  <tr>
    <td>Washington</td>
    <td>London</td>
  </tr>
  <tr>
    <td>Phoenix</td>
    <td>Arizona</td>
  </tr>
  <tr>
    <td>Austin</td>
    <td>Travis</td>
  </tr>
</table>
```

Tables which are used purely for layout do not need to do this. However, 
[the use of tables for layout is strongly discouraged](https://www.lifewire.com/dont-use-tables-for-layout-3468941).
This check will ignore tables that it believes are purposely used for layout.

Learn more about [accessible table design (W3C)](https://www.w3.org/WAI/tutorials/tables/one-header/).

add-header-to-tables

People using screen readers are not able to see the layout of a form. To make forms
accessible, they must define explicit text labels for each form control. 

Usually the best solution is to use a `<label>` element. The label may be linked to 
by the form control:

```html
<label for="name">Full name</label>
<input type="name" id="name">
```

or the `<label>` can be wrapped around the form control:

```html
<label>
    Full name <input type="name">
</label>
```

Buttons are different, as their labels are specified by the code for the button, e.g.

```html
<input type="submit" value="Send message">
<button>Send message</button>
```

Alternatively ARIA attributes, such as `aria-label` may be used, but this information
will not be conveyed to visual users. For more information, see [W3C's guide to labeling controls](https://www.w3.org/WAI/tutorials/forms/labels/).

Hidden input fields (`<input type="hidden">`) do not require labels.
Note that the `placeholder` attribute 
[should not be used as an alternative to a label](https://www.w3.org/TR/html/sec-forms.html#the-placeholder-attribute),
nor should the `title` attribute.

## Custom roles

Note that this check considers the underlying accessible role of an element, which can be 
different from the tag itself.

For example, this `<div>` has a `role` of `button`, and so is expected to have a label, like
a button would.

```html
<div role="button"></div>
```

form-control-labels

To comply with WCAG 2.1, fields must identify what their purpose is _programmatically_. If done correctly,
this allows browsers to help users fill in forms with known information, such as their name and email address.

This is accomplished via the `autocomplete` attribute, which should be set to a 
[supported value](https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/autocomplete) wherever
appropriate, e.g. 

```html
<label for="name">
  First Name
</label>
<input id="name" type="text" autocomplete="given-name">
```

For fields where `autocomplete` is not appropriate, e.g. due to security, or the field not matching the
specification, the specific field should be ignored.

## Methodology

* All fields which could support `autocomplete` (e.g. text fields)
  are checked. Other fields - like dropdown menus - are ignored.
* The [accessible name](https://www.w3.org/TR/accname-1.2/) of each field is considered. In most cases this
  will be the `<label>` for the field.
* If the name matches a known pattern the field 
  must have an appropriate `autocomplete` value specified.
  For example, an `<input>` with type `text` with a label of 
  "First Name" would be expected to have an autocomplete value 
  of `given-name` or similar.

Note that using `autocomplete="on"` is not sufficient to pass this check, a
specific named autocomplete value must be used. See the
[supported values](https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/autocomplete).

## Troubleshooting

If a field is a false positive, check whether the field has an 
appropriate accessible name (i.e. a `<label>` or `aria-label`).

If the label is incorrect, Silktide may assume that the field 
requires `autocomplete` where it does not. You should fix the 
label to avoid this.

Otherwise, you can simply ignore the specific field.

## More information

* [How to use HTML 5.2 attributes](https://www.w3.org/WAI/WCAG21/Techniques/html/H98)

identify-input-purpose

When specifying accessible names via `aria-labelledby`, ensure that the 
label elements exist.

## Explanation

Accessible names are required for many parts of a page, including form controls.
One way to accomplish this is to use `aria-labelledby` to specify the ID or IDs of 
other elements which contain a label. For example:

```html
<h2 id="dialog-labell">Important message!</h2>
<dialog aria-labelledby="dialog-label">
```

In this example, the `aria-labelledby` for the `<dialog>` points to an ID of `dialog-label`,
which doesn't exist. The heading it refers to instead has an ID of `dialog-labell`, presumably 
a typo.

## Why this matters

It isn't usually clear when an accessible name is broken, even during user testing.

This check helps to identify where an element clearly intended to have a label, but
that label is missing.

When a label is broken, it may be impossible for a user of assistive technologies
to understand what that element does.

missing-aria-labelled-by

Webpages should provide a way to skip to the main content of the page. There are 
several ways to accomplish this.

The most common solution is a "Skip to content" link:

```html
<a href="#main">Skip to main content</a>
...
<div id="main">
<h1>Welcome to this page</h1>
```

This link is usually hidden from users unless selected, so it doesn't need to compromise the look of 
your page.

Alternatively, you can use the `<main>` HTML landmark:

```html
<main>
<h1>Welcome to this page</h1>
``` 

or an ARIA role of `main`:

```html
<div role="main">
<h1>Welcome to this page</h1>
``` 

Generally best practice is to combine these approaches, using `<main>` landmark with a link pointing to it:

```html
<a href="#main">Skip to main content</a>
...
<main id="main">
<h1>Welcome to this page</h1>
```

This approach will be most widely supported by screen readers and understood by the highest number of users.

# Justification

Many users of assistive technology are only able to experience content sequentially - that is, they must
step through each part of the page, one part at a time. Often this is a slow process, potentially 
hampered by a physical impediment.

Most web pages have a header containing branding and navigation on every page. Stepping through 
this repetitive part of the page can be hugely frustrating for users of assistive technology. For example, 
a blind user may have to listen to hundreds of navigation links before they can reach the content on a page.

Adding a means to skip directly to the page content massively enhances the experience for these users.

Skip to content links do not need to be visible to everyone. A common implementation means they are only 
experienced by users via the tab key or other assistive technology 
(see [skipping to navigation links](https://webaim.org/techniques/skipnav/) from WebAIM). 

# Methodology

It is not sufficient to include a "skip to content" link. The link must work, and it must appear early
enough in the page to be useful.

* If the page has less than 500 visible characters, it does not require a "skip to content" link.
* If the page has a main landmark (e.g. `<main>` or `role="main"`) it does not require a "skip to content" link.
* Silktide looks for a link with a fragment (e.g. `#main`) pointing to a lower part of the page. The link 
  must not be adjacent to other text nodes. 
* The fragment must point to an ID that exists, or this check will raise an error.
* The link must appear before 500 visible characters of text in the page, or this check will raise an error.

The text inside the link is not considered, as this test must support a wide range of languages.

# Learn more

More about [skipping to navigation links](https://webaim.org/techniques/skipnav/) from WebAIM and 
see [W3C's recommended technique](https://www.w3.org/TR/2016/NOTE-WCAG20-TECHS-20161007/G1).

skip-to-content

Each page should have a well-defined page title (as defined by a single `<title>` tag).

add-page-title

When viewed on small screens (like mobile phones) content should be large enough to 
read without being zoomed-into and scrolled in two dimensions.

WCAG 2.1 specifically requires this for:

+ Vertical scrolling content at a width equivalent to 320 CSS pixels
+ Horizontal scrolling content at a height equivalent to 256 CSS pixels

This issue almost always indicates that the webpage has not been designed
to work on mobile devices at all.

extreme-zoom-scrolling

Every page should precisely specify what language(s) it is written in. 

## Why this matters

Screen readers, which read out the text on a page, need to know the language to 
pronounce it correctly. Without the language, text may sound like gibberish. 

For example: imagine a computer pronouncing just one German word
"die" (pronounced "dee") using English pronunciation. Now extend that to every
word on the page.

Search engines may also use the language to classify pages, so this can have
an effect on SEO as well.

## How to fix

The `<html>` element should have a `lang` or `xml:lang` attribute which describes 
the language of the page.

You will usually want to use the `lang` attribute as follows:

```html
<html lang="en-US">
```

(Note this example won't work for XHTML, which is deprecated and highly discouraged).

Make sure you specify a [valid language code](http://www.rfc-editor.org/rfc/bcp/bcp47.txt).

## More information

See [Technique H57](https://www.w3.org/TR/2016/NOTE-WCAG20-TECHS-20161007/H57).

html-lang-required

An `<img>` element inside a link must not specify alternative text that duplicates the
content of a text link inside or beside it.

For example, this code should be avoided:

```html
<a href="/apple">
    <img src="apple.png" alt="Apple">
    Apple
</a>
```

Accessible technologies will tend to see this as "Apple Apple". The correct behaviour is to
specify either a null (empty) `alt` attribute:

```html
<a href="/apple">
    <img src="apple.png" alt="">
    Apple
</a>
```

or to hide the image via `aria-hidden`:

```html
<a href="/apple">
    <img src="apple.png" aria-hidden="true">
    Apple
</a>
```

# Learn more

Read about [Technique H2](https://www.w3.org/TR/WCAG20-TECHS/H2) on how to combine
an adjacent image and texts links. 

See the [WAI Tutorial on decorative images](https://www.w3.org/WAI/tutorials/images/decorative/).

img-duplicate-alt

All controls on the page - including links, buttons, and fields - should clearly 
indicate when they have been selected by the keyboard.

This is a stricter version of [WCAG 2.4.7](https://www.w3.org/WAI/WCAG21/Understanding/focus-visible.html),
which requires that the focus indicator is visible, but doesn't specify how clearly
it should be visible.

You can confirm this yourself by loading a webpage and using the Tab key to navigate 
through it. Anything you can select should clearly alter its appearance when selected. 
For example, a text field might change color, or have a distinctive border applied. 

## The requirements

The [official requirements](https://w3c.github.io/wcag/understanding/focus-appearance.html) are 
complicated. A simplified version is:

* Surround the control with a solid outline
* The outline must have at least 3:1 contrast with its background
* The outline must have at least 3:1 contrast with its surroundings, or be at least 2px thick

The actual requirements allow for more variation, but generally require that you 
calculate the perimeter and/or area of the focus area, which can be very difficult.

## How to fix

Links and form controls should use a focus selector which contrasts with their
unfocused state, as well as any surroundings of the focus state. 

In practice, the easiest way to accomplish this is to use a two-color focus, like so:

```css
/* ensure high-contrast mode still has an indicator */
outline: 2px transparent solid;

/* Apply a thick yellow box-shadow with 
   a thin dark blue indicator in the middle */
box-shadow: 0 0 0 2px #F9F9D1, 0 0 0 4px #396196, 0 0 4px 8px #F9F9D1;
``` 

From [Technique C40](https://www.w3.org/WAI/WCAG22/Techniques/css/C40).

Using two contrasting colors guarantees that the focus state will be visible, 
regardless of the background or surrounding colors.

## Troubleshooting

### Not enough contrasting pixels change

The [Success Criterion](https://w3c.github.io/wcag/understanding/focus-appearance.html) specifies a minimum area 
for the contrasting portion of the focus indicator, which is equivalent to a 2 CSS pixel border around the component.

From the requirements:

> "The easiest and most common way to meet this requirement is to use a solid outline around the component. 
> The outline must be at least 2px thick."

In practice, many websites use alternatives to this, such as a thick border along one edge. A problem with 
that approach is it is often dependent upon the length of that one edge, which for text links can vary dynamically.

See [Sara Soueidan](https://www.sarasoueidan.com/blog/focus-indicators/#2.-minimum-contrasting-area) for a good
write-up of this concept.

To comply with this Success Criterion, we recommend adding a 2px border around all edges.

Note an earlier draft of this Success Criterion has a more generous allowance here, which is not the case for the
final version.

### Elements appear to change in the browser

Focus states should be defined in CSS alone, and not require JavaScript to work. 

Try disabling JavaScript on the page, and see if focus is still visible. If it not, 
then the focus state won't work with assistive technologies that don't have a physical
keyboard, e.g. mobile phones.

Some more unusual means of implementing a focus state cannot be detected or measured
automatically. These include:

* Focus implemented via pseudo-elements - such as `::after` or `::before`. 
* Focus using `:focus-within` (JavaScript current provides no way to access these).
* Focus cascaded to child elements, e.g. a `<span>` inside an `<a>` tag. We detect
  some of these cases, but there are variations that we cannot detect.

Currently, there is no practical way to assess these focus states automatically.

### Elements over images not analyzed

Silktide cannot assess the contrast of elements that are over images. This is because
the contrast of an element is calculated by comparing the color of the element with the
color of the background. 

If the element is over an image, the background color is the
color of the image, which varies from pixel-to-pixel, and may vary further as the page
is resized, due to responsive design.

### Unexpected elements are focusable

You may be surprised to see tags like `<div>` or `<span>` being shown as focusable.
If an element has a `tabindex` of 0 or higher, it becomes focusable. Any HTML tag can be made 
focusable in this way, this is so that custom controls (e.g. your own drop-down menus, or 
sliders) can support keyboard navigation. [Learn more about tabindex on MDN](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/tabindex).

## Further reading

* [WCAG 2.4.13](https://www.w3.org/WAI/WCAG22/Understanding/focus-appearance.html)
* [Minimum Contrasting Area explained (Sara Soueidan)](https://www.sarasoueidan.com/blog/focus-indicators/#2.-minimum-contrasting-area)

ensure-focus-appearance

An image with empty alternative text (e.g. `<img alt="">`) should not specify a 
`title` attribute, or if specified, the `title` should be empty.

For example, this is invalid:

```html
<img src="apple.png" alt="" title="This is an apple">
```

img-with-empty-alt-but-has-title

When viewed on small screens (like mobile phones) content must fit without 
requiring scrolling in two dimensions.

WCAG 2.1 specifically requires this for:

+ Vertical scrolling content at a width equivalent to 320 CSS pixels
+ Horizontal scrolling content at a height equivalent to 256 CSS pixels

Except for parts of the content which require two-dimensional layout for usage or meaning,
such as a maps application.

Priority	Recommendation	Progress
	Ensure focus is not fully obscured WCAG 2.2 AA 2.4.11	10%
	Ensure content is not too difficult to understand WCAG 2.0 AAA 3.1.5	54.1%
	Ensure focus is not partly obscured WCAG 2.2 AAA 2.4.12	10%
	Ensure interactive components are far enough apart WCAG 2.2 AA 2.5.8	92%
	Ensure every page contains a top-level heading WCAG 2.0 A 1.3.1	77%

KTH Royal Institute of Technology

Accessibility

WCAG Levels

Recommendations

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