There are several layout issues caused by rendering differences of sub-pixel values in various rendering engines.
Blurriness, width/height differences of same-height widgets side-by-side, not perfectly round circles (implemented by border-radius) are all caused by it.
The linked articles from 2009 & 2010 are not completely describing our issues, but providing some good background information on the source of the cause.

Possible solutions on trial to use:

1. rems instead of ems together with a normalization of root font size to 10px
   • Pro: If flexibility on scaling and on being accessible is needed, (respecting user stylesheet's`font-size` settings) while ensuring design is at spec at normal zoom levels, rem with pixel fallback is the value to go. MW core is currently relying on it.
   • Con: General implementation seems problematic, because we provide OOjs UI to MediaWiki installations where we can't rely on a certain root base font size.
2. CSS calc() as way out of sub-pixels
   • Pro: Might solve an IE specific bug introduced by miscalculation of subdecimal em values more than two digits, see T102364
   • Con: Lack of wide browser support. IE 9, Android up to 4.4.4, in short 77.24% of browser's have support currently. T123656 was addressed by using calc()
   • Con: Might run into sub-pixel values again, if not done carefully.
3. JS when layout computation is done in rendering engine and do rounding ourselves
   • Con: Might result in negative performance impact (f.e. repaint or flicker) and in general it doesn't seem like a good idea to take over browser's duty.
4. CSS min-width and min-height absolute values to ensure an full pixel value at 100% zoom
   • Pro: Could be applied to several widgets without major code bloat
   • Con: If used in custom MediaWiki with font-size below OOjs UI default, widgets might result larger in proportion to context. That is becoming an issue with a body font-size below 12px.
   • Note: IE has various issues with min-width/min-height in ems.
5. Less way to let browser calculate to rounded full-pixel values by using base font-size. Approach taken in https://gerrit.wikimedia.org/r/#/c/315456/
   • Pro: Output is well rounded and design templates are easier to follow-up. It's more intuitive for developers than only relying on em
   • Con: Short outputs like 1em or 0.5em aren't used any more, although the computed value is nice, the stylesheet value isn't as appealing.