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This is an early unpublished editor's draft; content is incomplete and subject to change.

No flashing (no exceptions)

supplemental

Normative Text

Content does not include flashing.

Method(s)

  • Design content without using flashing.
Tests

This section is non-normative.

Procedure

For each page/view:

  1. Check if content includes flashing.

Expected results

  • #1 is false.

Tests

This content needs to be written.

Key Terms

blinking

switching back and forth between two visual states in a way that is meant to draw attention

See also flash. It is possible for something to be large enough and blink brightly enough at the right frequency to be also classified as a flash.

content

information, sensory experience and interactions conveyed

flash

a pair of opposing changes in relative luminance that can cause seizures in some people if it is large enough and in the right frequency range

See general flash and red flash thresholds for information about types of flash that are not allowed.

See also blinking.

general flash and red flash thresholds

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

  • there are no more than three general flashes and / or no more than three red flashes within any one-second period; or
  • 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 (1.0) 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

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.

For general software or web content, using a 341 x 256 pixel rectangle anywhere on the displayed screen area when the content is viewed at 1024 x 768 pixels will provide a good estimate of a 10 degree visual field for standard screen sizes and viewing distances (e.g., 15-17 inch screen at 22-26 inches). This resolution of 75 - 85 ppi is known to be lower, and thus more conservative than the nominal CSS pixel resolution of 96 ppi in CSS specifications. Higher resolutions displays showing the same rendering of the content yield smaller and safer images so it is lower resolutions that are used to define the thresholds.

A transition is the change in relative luminance (or relative luminance/color for red flashing) between adjacent peaks and valleys in a plot of relative luminance (or relative luminance/color for red flashing) measurement against time. A flash consists of two opposing transitions.

The new working definition in the field for “pair of opposing transitions involving a saturated red” (from WCAG 2.2) is a pair of opposing transitions where, one transition is either to or from a state with a value R/(R + G + B) that is greater than or equal to 0.8, and the difference between states is more than 0.2 (unitless) in the CIE 1976 UCS chromaticity diagram. [[ISO_9241-391]]

Tools are available that will carry out analysis from video screen capture. However, no tool is necessary to evaluate for this condition if flashing is less than or equal to 3 flashes in any one second. Content automatically passes (see #1 and #2 above).

relative luminance

the relative brightness of any point in a colorspace, normalized to 0 for darkest black and 1 for lightest white

For the sRGB colorspace, the relative luminance of a color is defined as L = 0.2126 * R + 0.7152 * G + 0.0722 * B where R, G and B are defined as:

  • if RsRGB <= 0.04045 then R = RsRGB/12.92 else R = ((RsRGB+0.055)/1.055) ^ 2.4
  • if GsRGB <= 0.04045 then G = GsRGB/12.92 else G = ((GsRGB+0.055)/1.055) ^ 2.4
  • if BsRGB <= 0.04045 then B = BsRGB/12.92 else B = ((BsRGB+0.055)/1.055) ^ 2.4

and RsRGB, GsRGB, and BsRGB are defined as:

  • RsRGB = R8bit/255
  • GsRGB = G8bit/255
  • BsRGB = B8bit/255

The ”^” character is the exponentiation operator. (Formula taken from [[SRGB]].)

Before May 2021 the value of 0.04045 in the definition was different (0.03928). It was taken from an older version of the specification and has been updated. It has no practical effect on the calculations in the context of these guidelines.

Almost all systems used today to view web content assume sRGB encoding. Unless it is known that another color space will be used to process and display the content, authors should evaluate using sRGB colorspace.

If dithering occurs after delivery, then the source color value is used. For colors that are dithered at the source, the average values of the colors that are dithered should be used (average R, average G, and average B).

Tools are available that automatically do the calculations when testing contrast and flash.

WCAG 2.2 includes a separate page giving the relative luminance definition using MathML to display the formulas. This will need to be addressed for inclusion in WCAG 3.