https://developer.mozilla.org/zh-CN/docs/Web/CSS/Using_CSS_variables
https://css-tricks.com/difference-between-types-of-css-variables/
http://www.ruanyifeng.com/blog/2017/05/css-variables.html
http://www.ruanyifeng.com/blog/2017/05/css-variables.html
https://css-tricks.com/making-custom-properties-css-variables-dynamic/
https://scotch.io/tutorials/improve-the-performance-of-your-javascript-web-application
https://danielcwilson.com/blog/2017/02/individual-transforms/
http://www.ruanyifeng.com/blog/2017/04/css_in_js.html
http://www.ruanyifeng.com/blog/2017/03/boundary.html
http://www.ruanyifeng.com/blog/2017/05/fish_shell.html
http://www.ruanyifeng.com/blog/2017/04/memory-leak.html
currentColor
关键字https://developer.mozilla.org/zh-CN/docs/Web/CSS/color_value#currentColor_关键字
https://osvaldas.info/keeping-css-short-with-currentcolor
https://osvaldas.info/keeping-css-short-with-currentcolor
https://css-tricks.com/currentcolor/
http://thenewcode.com/blog/908/The-First-CSS-Variable-currentColor
https://osvaldas.info/keeping-css-short-with-currentcolor
css var
currentcolor
http://caniuse.com/#search=currentcolor
https://developer.mozilla.org/zh-CN/docs/Web/CSS/color_value#currentColor_关键字