[1] Selectors ( ( 版)) <http://www.w3.org/TR/1998/WD-css2-19980128/selector.html#h-5.7.1.1>
[2] Fix #991: Define case-sensitivity of the argument to attr() · whatwg/html@fb8ed61 ( 版) <https://github.com/whatwg/html/commit/fb8ed61b69109b14dc8ffac5dbca6d04aab28ca8>
[3] [css-values] Generify the handling of units in the attr() type arg. · w3c/csswg-drafts@1741f95 ( 版) <https://github.com/w3c/csswg-drafts/commit/1741f95fc91ef2309daa5a1000a4d7b0197c652a>
[4] 435426 - implement css3-values extensions to attr() () <https://bugzilla.mozilla.org/show_bug.cgi?id=435426>
[5] 246571 - Implement CSS3 attribute / attr references - chromium - Monorail () <https://bugs.chromium.org/p/chromium/issues/detail?id=246571>
[6] Use CSS Syntax tokens for attr() (csnardi著, ) <https://github.com/w3c/csswg-drafts/commit/d65e80d0dcec8b8943a4844b24cad42c79d53de2>
[7] Use CSS Syntax tokens for attr() by csnardi · Pull Request #2293 · w3c/csswg-drafts () <https://github.com/w3c/csswg-drafts/pull/2293>