[css-regions][css-values] font-size in em, and fragments

Hi,

While I was working on some [css-regions] demo (1), I noticed that the "pull-quote text-negociation" didn't work anymore.

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Use case: You have a region and you adjust its font-size to avoid it to overflow/overset. 

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In previous versions of Blink/Webkit, it used to work because the EM unit was computed relatively to the layout tree (so fragments inside a region with "font-size: 2em" where actually bigger than those in a region with "font-size: 1em".

Now, it seems this has been "fixed" and the EM unit is relative to the DOM parent. That means that:

- if you have a TEXTNODE directly poured into a flow, its FONT-SIZE WILL DEPEND ON THE REGION.
- if you have an ELEMENT directly poured into a a flow, its FONT-SIZE WILL NOT DEPEND ON THE REGION because the EM value will be computed on the basis on the DOM parent element.

This inconsistency is damageable, and I therefore propose to make sure the EM unit in a region fragment is relative to the region itself, not the actual DOM.





Thoughts?





(1) https://rawgithub.com/adobe-webplatform/css-regions-polyfill/master/playground/paginated-content/index.html
------ only works in Chrome Canary 
------ and with Experimental features enabled