[1] 構文解析時?に em に変換されるみたいです。

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

[3] 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 (2016-04-05 14:04:40 +09:00 版) https://www.w3.org/Bugs/Public/show_bug.cgi?id=27673