Use rem units for font sizes so that font sizes are relative to the root
<html> element, not the parent element.
Fixes an issue where the video duration indicator would be too small on
parent/child thumbnails in post show pages. This was because of nesting
issues with em units. Em units are relative to their parent element, so
if you had a parent element with a font size of 0.8em, and a child
element with a font size of 0.8em, then the final computed font size
would be 0.8*0.8 = 0.64em.
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