[2] css - What does 1__qem mean? - Stack Overflow ( ( 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 ( 版) <https://www.w3.org/Bugs/Public/show_bug.cgi?id=27673>