Follow-up of #4599 ; @tabatkins had suggested me to check other "case-insensitive" parts. I'm copying what I said in #4599 (comment)

• "All Selectors syntax is case-insensitive within the ASCII range (i.e. [a-z] and [A-Z] are equivalent), except for parts that are not under the control of Selectors." https://www.w3.org/TR/selectors-3/#casesens => For non-exception this is just ASCII-case insensitiveness
• "The matching of C against the element's language value is performed case-insensitively within the ASCII range. " https://www.w3.org/TR/selectors-3/#lang-pseudo => Same here.
• " Pseudo-class names are case-insensitive. " https://www.w3.org/TR/selectors-3/#pseudo-classes => I wonder if this can be restricted to ASCII case-insensitive?
• "CSS style sheets are generally case-insensitive, and this is also the case for media queries. " https://www.w3.org/TR/css3-mediaqueries/#syntax => Same here.