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

provided changes to address patrick's comments.

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

@@ -44,7 +44,9 @@ <h2>Intent of Three Flashes or Below Threshold</h2>
          represents the central vision portion of the eye, where people are most susceptible
          to photo stimuli.)
       </p>
-      <p>With the proliferation of mobile and devices of many sizes, 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 as a density-independent unit of measurement, the prior assessment criteria seem outdated.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 assessment becomes 341 x 256 CSS pixels, or a flashing area of 87,296 CSS pixels.</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 as a density-independent unit of measurement, the prior assessment criteria seem outdated.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 assessment 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