[csswg-drafts] [css-values] Fallback value of ex height (#5243)

fantasai has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-values] Fallback value of ex height ==
In https://github.com/w3c/csswg-drafts/issues/814#issuecomment-422414048 @faceless2  wrote: 

> Another quick addendum as this doesn't warrant a post on it's own - but as I was just banging on about font metrics for 100 lines, it's relevant to note that the other significant metric extracted from the font is x-height, used for the "ex" unit and middle alignment. This is extracted from "OS2.sxHeight. If it is zero, Firefox, Chrome and Safari all set it to 0.8em - not the 0.5em currently recommended in css-values-4.

which does in fact deserve a post on its own. :)

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/5243 using your GitHub account