Flicker Guidelines v1.00
1. WAI No-flicker requirement
1.1. You SHOULD NOT put any dynamic content which visibly or purposely flickers between 3 and 49 Hz (i.e. flashes more than once or twice a second) onto a page. Whilst visible flicker may depend on the user's system capabilities, and thus may not be able to be tested thoroughly before launch, you MUST fix any content which validated feedback from users says flickers. [based on WAIv2, level 2]
1.2. If flicker is unavoidable, you MUST warn the user before they go to the content with the flicker, and where editorially appropriate, provide an alternative version of the content without the flicker which is as close to the original as is possible. By implication, this means that top-level-directory (tld) pages MUST NOT flicker. [based on WAIv2, level 2]
2. Flicker warning
2.1. For pages:
If you place a link to a page which flickers on your page, you MUST follow the link immediately with the following warning:
'Please note that this link takes you to a page which includes materials that flicker and so may not be suitable for users with photosensitive epilepsy.'
2.2. For games:
If your game includes content that flickers, you MUST place the following warning on the game's title screen (before the user is exposed to the flickering content):
'Please note that this game includes materials that flicker and so may not be suitable for users with photosensitive epilepsy.'
3. Definition of 'flicker'
Like the WAI Guidelines, these guidelines do not attempt to define what "flicker" is. Our current understanding is that moving content is not impacted by these requirements, but only moving content that "flickers" – quickly and continually switches between a light and dark colour, like a strobe light.
4. How to check your dynamic content
To check that your dynamic content does not flicker between 3 and 49 Hz, make a visual comparison of your content against the NM S&G Flicker Tool (contact your BBC Technical Account Manager for access) – the W3C have no tool to offer.
In cases where it is not clear if your content passes or fails this test, you should send a link to your content to the Editor, NM Standards & Guidelines for advice.
5. Reasoning & background
These requirements are based on v2 of the W3C WAI Guidelines (the latest and most comprehensive version) – w3.org/Web Content Accessibility Guidelines\Avoid Causing the Screen to Flicker
Individuals with photosensitive epilepsy can have seizures triggered by flickering or flashing in the 3 to 49 flashes per second (Hertz) range with a peak sensitivity at 20 flashes per second.
Individuals with distractibility problems may not be able to focus on page content with flicker occurring in the same visual field.
6. Triggers for updating this document
- We will update the definition of flicker in this document when a better one is available from the W3C or epileptic groups.
- We will continue to develop the NM S&G Flicker Tool as our experience of using it grows, until such point as a more practical tool is identified.
- We will review these guidelines six months after their approval date, when we have better experience of judging what constitutes "flicker".
7. Document history
| Date | Version | Change | Author |
|---|---|---|---|
| 26/09/2003 | v1.00 | Standard renumbered as 1.00 on approval by Standards Exec | Jonathan Hassell |
| 23/07/2003 | v0.3 | After alterations required by Tech Forum to approve doc (on 22/07/2003 ) | Jonathan Hassell |
| 18/07/2003 | v0.2 | Second draft of the doc, with alterations required by Accessibility WG to approve it (see minutes of meeting on 09/07/2003 ) | Jonathan Hassell |
| 08/07/2003 | v0.1 | First draft of this doc, based on discussion at Accessibility WG meeting on 11/06/2003 | Jonathan Hassell |
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Like all other Future Media Standards & Guidelines, this page is updated on a regular basis, through the process described on About Standards & Guidelines.
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