User agents and operating systems should ensure that a CSS pixel is
      set as closely as possible to the CSS Values and Units Module Level 3 reference pixel [ css3-values ], which takes into account the physical dimensions of the display
      and the assumed viewing distance (factors that cannot be determined by content authors). down-event platform event that occurs  when the trigger stimulus of a pointer is depressed The down-event may have different names on different platforms, such as "touchstart" or "mousedown". dragging movement [New] an operation where the pointer engages with an element on the down-event and the element (or a representation of its position) follows the pointer until an up-event Note Examples of draggable elements include list items, text elements, and images. emergency a sudden, unexpected situation or occurrence that requires immediate action to preserve
      health, safety, or property encloses [New] solidly bounds or surrounds essential if removed, would fundamentally change the information or functionality of the content, and information and functionality cannot be achieved in another way that would conform extended audio description audio description that is added to an audiovisual presentation by pausing the video so that there is time to add additional description Note This technique is only used when the sense of the video would be lost without the additional audio description and the pauses between dialogue/narration are too short. 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 Note 1 See general flash and red flash thresholds for information about types of flash that are not allowed.