[1] 構文解析時?に [CODE(CSS)@en[[[em]]]] に変換されるみたいです。

[2] [CITE[css - What does 1__qem mean? - Stack Overflow]]
( ([TIME[2013-08-27 02:38:35 +09:00]] 版))
<http://stackoverflow.com/questions/7448673/what-does-1-qem-mean>

[3] [CITE@en[27673 – margins in this section need LTR-specific note added. margin-top/bottom might also not be correct for vertical. See https://github.com/domenic/html-as-custom-elements/issues/40]]
([TIME[2016-04-05 14:04:40 +09:00]] 版)
<https://www.w3.org/Bugs/Public/show_bug.cgi?id=27673>