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Success Criterion 2.3.1: Three Flashes or Below Threshold (Level A)

Web pages do not contain anything that flashes more than three times in any one second period, or the flash is below the general flash and red flash thresholds.

Note: Since any content that does not meet this success criterion can interfere with a user's ability to use the whole page, all content on the Web page (whether or not it is used to meet other success criteria) must meet this success criterion. See Conformance Requirement 5: Non-Interference.

(From Understanding WCAG 2.0: Understanding Success Criterion 2.3.1)

Note

Superseded by Success Criterion 2.3.2 at level AAA. That Success Criterion removes the flash threshold allowances.

Discussion

Table of Contents

  1. Definition of a flash
  2. So what does this mean?
    1. What is "small enough"?
  3. Comparison with WCAG 1.0
  4. Comparison with U.S. Section 508

Definition of a flash

The definitions of a "general flash" or a "red flash" are defined below in the threshold definition:

a flash or rapidly changing image sequence is below the threshold (i.e., content passes) if any of the following are true:

  1. there are no more than three general flashes and / or no more than three red flashes within any one-second period; or
  2. the combined area of flashes occurring concurrently occupies no more than a total of .006 steradians within any 10 degree visual field on the screen (25% of any 10 degree visual field on the screen) at typical viewing distance

where:

  • A general flash is defined as a pair of opposing changes in relative luminance of 10% or more of the maximum relative luminance where the relative luminance of the darker image is below 0.80; and where "a pair of opposing changes" is an increase followed by a decrease, or a decrease followed by an increase, and
  • A red flash is defined as any pair of opposing transitions involving a saturated red. [note: check the full definition for the definition of a "saturated red"]

Exception: Flashing that is a fine, balanced, pattern such as white noise or an alternating checkerboard pattern with "squares" smaller than 0.1 degree (of visual field at typical viewing distance) on a side does not violate the thresholds.

WCAG 2.0 Recommendation: "general flash and red flash thresholds" definition

So what does this mean?

This definition is one of the most complex found in the WCAG 2.0 definitions, so it is worth breaking down. The easiest method to pass this Success Criterion, of course, is to ensure that flashes in content do not occur more often than three times per second. Otherwise, at A level, an exception is offered where content will pass if the combined area of flashing on the screen is "small enough".

What is "small enough"?

The definition of small enough is given as "25% of any 10 degree visual field on the screen... at typical viewing distance". This size, therefore, depends on the screen size, resolution, and viewing distance. The higher the resolution or screen size, the larger the permitted area. Where content is to be viewed on a known monitor (such as a kiosk, digital billboard, or other standard operating environment), the W3C provide a formula for calculating the "small safe area" for known displays.

For general web usage, however, a baseline needs to be established based on a common resolution. When the recommendation was made final in late 2008, the baseline was set to a 15-17 inch monitor at 1024 x 768 resolution, viewed at 22-26 inches (55-65 cm). For such a baseline, the "small safe area" is a contiguous region of 21,824 pixels squared (or a quarter of a 341 x 256 pixel rectangle). This covers screens at higher resolutions and screen sizes, as the "small safe area" would cover a lesser perce3ntage of such field of vision.

Comparison with WCAG 1.0

This Success Criterion replaces a stricter no-flash or "no-flicker" requirement in WCAG 1.0 (Checkpoint 7.1) which applied at A level. WCAG 1.0 defined "flickering" as between 4 and 59 flashes per second.

A strict no-flash requirement still applies at AAA level as Success Criterion 2.3.2.

Comparison with U.S. Section 508

Those authors wishing to comply with the current Section 508 requirements for web sites must avoid flashing within the range of 2 and 55 flashes per second, not inclusive. (Section 508 Standards Guide - Web-based information/applications section, rule J)

In the current (very slowly-moving) process of updating Section 508, it is proposed that these requirements will be replaced with that of WCAG 2.0 Level AA. If authors comply with the current Section 508 requirement by restricting flashes to two per second will also comply with Success Criterion 2.3.1, and therefore the proposed update to Section 508.

Sniff Coverage

Note that "[Standard]" will be replaced with "WCAG2A", "WCAG2AA" or "WCAG2AAA", depending on the level being tested.

Notices

Message Name Applies To Techniques

Check that no component of the content flashes more than three times in any 1-second period, or that the size of any flashing area is sufficiently small.

[Standard].Principle2.Guideline2_3.2_3_1.G19,G176

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