[1] calc() - CSS | MDN ( ()) https://developer.mozilla.org/en/docs/Web/CSS/calc
[2] [css3-values] dropping min()/max() on calc() ( (L. David Baron著, )) https://lists.w3.org/Archives/Public/www-style/2010Sep/0233.html
[3] [css-values] Allow division of same types in calc() · Issue #545 · w3c/csswg-drafts () https://github.com/w3c/csswg-drafts/issues/545