End to End analysis of Guideline 2.3

Analysis of the table below:

There are no HTML techniques for this. The CSS Technique has an "AND" relationship to 2.2. In other words it would apply to both 2.2 & 2.3.

  Guideline Technology Independent
Doc		 HTML Techniques CSS
Guideline 2.3
2.3: Allow users to avoid content that could cause photosensitive epileptic seizures. 		     
Success Criteria 1:

Level 1 Success Criteria for Guideline 2.3

  1. Content that violates General Flash Threshold or Red Flash Threshold is marked in way that the user can access prior to its appearance. [V]

 

2.3.1 Flicker

This technique satisfies guideline(s):

  • Guideline 2.3 Allow users to avoid content that could cause photosensitive epileptic seizures.

Task:

Ensure that users can avoid experiencing screen flicker.

A flickering or flashing screen may cause seizures in users with photosensitive epilepsy and content developers should thus avoid causing the screen to flicker. Seizures can be triggered by flickering or flashing in the 4 to 59 flashes per second (Hertz) range with a peak sensitivity at 20 flashes per second as well as quick changes from dark to light (like strobe lights).
Editorial Note: A lot of this is now in the Guidelines, need to discuss with the WCAG WG how much should here. Also need information on testing and a reference to Trace's new tool. (currently in the Guideline) depending on the resolution of the WG.

Guideline 2.4 Facilitate the ability of users to orient themselves and move within the content.

 

2.13 Text style effects

Task:

@@

The following CSS2 properties can be used to style text:
  • Case: 'text-transform' (for uppercase, lowercase, and capitalization).
  • Shadow effects: 'text-shadow'
  • Underlines, overlinks, blinking: 'text-decoration'.

Note. If blinking content (e.g., a headline that appears and disappears at regular intervals) is used, provide a mechanism for stopping the blinking. In CSS, 'text-decoration: blink' will cause content to blink and will allow users to stop the effect by turning off style sheets or overriding the rule in a user style sheet. Do not use the BLINK and MARQUEE elements. These elements are not part of any W3C specification for HTML (i.e., they are non-standard elements).

 
Success Criteria 2:

Level 2 Success Criteria for Guideline 2.3

  1. Content does not violate the General Flash Threshold or Red Flash Threshold. [V]
     
         
Success Criteria 3:

Level 3 Success Criteria for Guideline 2.3

  1. Content does not violate any of the Spatial Pattern Thresholds. [V]

General Flash Threshold

  • A sequence of flashes or rapidly changing image sequences is not permitted when both the following occur:
    1. the combined area of flashes occurring concurrently (but not necessarily contiguously) occupies more than one quarter of any 355 x 268 pixel rectangle anywhere on the displayed screen area when the content is viewed at 1024 by 768 pixels and
    2. there are more than three flashes within any one-second period.

    Note:

    For the general flash threshold, a flash is defined as a pair of opposing changes in luminance (i.e., an increase in luminance followed by a decrease, or a decrease followed by an increase) of 20 candelas per rectangle meter (cd.m-2) or more and where the screen luminance of the darker image is below 160 cd.m-2.

Red Flash Threshold

  • A transition to or from a saturated red at any luminance level is not permitted when both of the following occur:
    1. the combined area of flashes occurring concurrently occupies more than one quarter of any 355 x 268 pixel rectangle anywhere on the displayed screen area when the content is viewed at 1024 by 768 pixels and
    2. there are more than three flashes within any one-second period.

    Note:

    For the red flash threshold, a flash is defined as any pair of opposing transitions to or from a saturated red at any luminance level . (See notes 1 and 2 below)

Spatial Pattern Thresholds

  • Clearly discernible stripes consisting of more than five light-dark pairs in any orientation are not permitted when the stripes are stationary when the pattern occupies more than 40% of any 355 x 268 pixel rectangle anywhere on the displayed screen area when the content is viewed at 1024 by 768 pixels.
  • Clearly discernible stripes consisting of more than five light-dark pairs in any orientation that change direction, oscillate, flash, or reverse in contrast are not permitted when the pattern occupies more than 25% of any 355 x 268 pixel rectangle anywhere on the displayed screen area when the content is viewed at 1024 by 768 pixels.
    1. Clearly discernable stripes are stripes where the screen luminance of the darker bars in the pattern is below 160 cd.m-2 and differs from the lighter bars by 20 cd.m-2 or more (see notes 1 and 2 below).
     
Informative Guideline 2.3 (flicker) Issues

Notes:

  1. Video waveform luminance is not a direct measure of display screen brightness. Not all display devices have the same gamma characteristic, but a display with a gamma of 2.2 may be assumed for the purpose of determining electrical measurements made to check compliance with these guidelines.
  2. For the purpose of measurements made to check compliance with these guidelines, pictures are assumed to be displayed in accordance with the 'home viewing environment' described in Recommendation ITU-R BT.500 in which peak white corresponds to a screen illumination of 200 cd.m-2.
  3. Thresholds are based on ITC Guidance Note for Licensees on Flashing Images and Regular Patterns in Television (Revised and re-issued July 2001) as modified by the Wisconsin Computer Equivalence Algorithm.
Editorial Note: A free tool will be available from the University of Wisconsin's Trace Center that will carry out the above analysis on Web content. The tool will be available by the second quarter of 2004.

 

Who Benefits from Guideline 2.3 (Informative)

  • Individuals with photosensitive epilepsy can avoid having seizures triggered by flashing or by spatial patterns.