[csswg-drafts] [css-values] Font-relative length definitions don't account for media query usage

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

== [css-values] Font-relative length definitions don't account for 
media query usage ==
(All emphases added)

> [...] the [font-relative 
lengths](https://drafts.csswg.org/css-values/#font-relative-lengths) 
refer to the font metrics of the **element on which they are used**.
> [...]
> <dl><dt><a href="https://drafts.csswg.org/css-values/#em">em 
unit</a></dt>
> <dd>Equal to the computed value of the 'font-size' property of the 
<b>element on which it is used</b>.</dd></dl>

When using such font-relative lengths in a media query expression:
* The author isn't "using" the length "on" something, per se.
* In the most common case, the author is using the length in relation 
to the viewport, which isn't an *element*.

> <dl><dt><a href="https://drafts.csswg.org/css-values/#rem">rem 
unit<a></dt>
> <dd>Equal to the <b>computed value</b> of font-size on the root 
element. When specified on the 'font-size' property [...]</dd></dl>

Media queries interpret `rem` differently; see 
https://drafts.csswg.org/mediaqueries/#units

The other font-relative unit definitions likewise don't seem to 
account for usage in media query expressions.

Please view or discuss this issue at 
https://github.com/w3c/csswg-drafts/issues/153 using your GitHub 
account