Merge pull request #2127 from w3c/flashing-measurement

Update three-flashes-or-below-threshold.html

understanding/20/three-flashes-or-below-threshold.html

@@ -20,8 +20,8 @@ <h2>Intent of Three Flashes or Below Threshold</h2>
       <p>Individuals who have photosensitive seizure disorders can have a seizure triggered
          by content that flashes at certain frequencies for more than a few flashes. People
          are even more sensitive to red flashing than to other colors, so a special test is
-         provided for saturated red flashing. These guidelines are based on guidelines for
-         the broadcasting industry as adapted for computer screens, where content is viewed
+         provided for saturated red flashing. These guidelines were originally based on guidelines for
+         the broadcasting industry as adapted for desktop monitors, where content is viewed
          from a closer distance (using a larger angle of vision).
       </p>
 
@@ -38,12 +38,15 @@ <h2>Intent of Three Flashes or Below Threshold</h2>
          did not allow any flashing (even of a single pixel) within a broad frequency range
          (3 to 50 Hz). This Success Criterion is based on existing specifications in use in
          the UK and by others for television broadcast and has been adapted for computer display
-         viewing. The 1024 x 768 screen is used as the reference screen resolution for the
+         viewing. In WCAG 2.0, a 1024 x 768 screen was used as the reference screen resolution for the
          evaluation. The 341 x 256 pixel block represents a 10 degree viewport at a typical
          viewing distance. (The 10 degree field is taken from the original specifications and
          represents the central vision portion of the eye, where people are most susceptible
          to photo stimuli.)
       </p>
+      <p>With the proliferation of devices of varying screen sizes (from small hand-helds to large living room displays), as well as the adoption of <a href="https://www.w3.org/TR/WCAG21/#dfn-css-pixels">CSS pixels</a> as a density-independent unit of measurement, the prior assessment criteria may seem outdated. However, an image of a consistent size uses up relatively the same percentage of a user's visual field on any device. On a large screen, the image takes up less size, but the large screen takes up a larger part of the visual field. On a mobile screen, the image may take up most or all of the screen; however, the mobile screen itself takes up a smaller portion of the user's visual field. So the same dimension of the flashing content, represented in CSS pixels can still provide a consistent means of assessment. Substituting CSS pixels for the original pixel block means that the combined area of flashing becomes 341 x 256 CSS pixels, or a flashing area of 87,296 CSS pixels.</p>
+
+      <p class="note">The specification cannot account for the actual viewing distance that a person chooses. Users that are closer to their screens than the idealized viewing distance will be affected by flashing areas that normatively pass. The same problem applies to users who rely on zoom or screen magnification. Conversely, users who are further away from the screen than the idealized distance should be able to tolerate flashing areas that are larger than the threshold.</p>
 
       <p>The combined area of flashes occurring concurrently and contiguously means the total
          area that is actually flashing at the same time. It is calculated by adding up the