compare selector with tag name about case-sensitivity

[UnVuTWFu]

The spec says:

When comparing a CSS element type selector to the names of HTML elements in HTML documents, the CSS element type selector must first be converted to ASCII lowercase. The same selector when compared to other elements must be compared according to its original case. In both cases, to match, the values must be identical to each other (and therefore the comparison is case sensitive).

Is this mean CSS's syntax is lowercased, HTML's tag/attribute name is lowercased, and then a case-sensitive match is performed?If so,why not add a sentence to more clearly explain how it compare

[annevk]

What this is saying is that a FOO type selector will match an element FOO in the HTML namespace in XML documents, but not in HTML documents.

I created an example for the latter scenario: https://software.hixie.ch/utilities/js/live-dom-viewer/?%3C!DOCTYPE%20html%3E.%0A%3Cstyle%3EFOO%2C%20foo%20%7B%20background%3Ared%20!important%20%7D%3C%2Fstyle%3E%0A%3Cscript%3E%0Afoo%20%3D%20document.body.appendChild(document.createElementNS(%22http%3A%2F%2Fwww.w3.org%2F1999%2Fxhtml%22%2C%20%22FOO%22))%0Afoo.style%20%3D%20%22background%3Alime%22%3B%0Afoo.textContent%20%3D%20%22hello%22%3B%0A%3C%2Fscript%3E

[UnVuTWFu]

What this is saying is that a FOO type selector will match an element FOO in the HTML namespace in XML documents, but not in HTML documents.

Yes,I have known about it,but I really doubt is the definition of "comparison" is unclear.Accoring to the infra

A string a is or is identical to a string b if it consists of the same sequence of code units.

Except where otherwise stated, all string comparisons use is.

But the fact is,for example,foo type selector will math element foo,FOO or fOo.The comparison between the type selector and html elemenet in html document is actually case-insensitive.

I think should add a sentance to explain the html tag name should also be converted to ASCII lowercase when comparsion.In order to follow definition of is

[annevk]

It's not case-insensitive? I have just shown you it is not.

The HTML parser lowercases most elements, which is what makes this not observable most of them and might make people think it is case-insensitive. But if you insert elements yourself using createElementNS() you can observe it is in fact case-sensitive after a lowercase operation on the selector side.

[UnVuTWFu]

OK,I understand.

The HTML parser lowercases most elements

This is what bothers me，because I only read §13.1.2

Tags contain a tag name, giving the element's name. HTML elements all have names that only use ASCII alphanumerics. In the HTML syntax, tag names, even those for foreign elements, may be written with any mix of lower- and uppercase letters that, when converted to all-lowercase, matches the element's tag name; tag names are case-insensitive.

This sentance “tag names are case-insensitive" makes me misunderstand.The context of “when converted to all-lowercase” is also unclear.

And I don't read §13.2.5.8 Tag name state

ASCII upper alpha
Append the lowercase version of the current input character (add 0x0020 to the character's code point) to the current tag token's tag name.

This explains the tag name will lowercased after tokenization.So when compare it to type selector,it needn't to lowercased.The attribute name and attribute selector is same as that.

But attribute value selector is not like this.It keeps its original case.Some attribute value is compared with value selector should be ASCII case-insensitive according to

Attribute selectors on an HTML element in an HTML document must treat the values of attributes with the following names as ASCII case-insensitive:

That's what I understand now.I have to say that the original text is somewhat misleading.There must be something can be improved.Maybe clarify the process of comparison,or have a more clear definition of ‘case-insensitive’ and ‘case-sensitive’.

[tabatkins]

This sentance “tag names are case-insensitive" makes me misunderstand.The context of “when converted to all-lowercase” is also unclear.

Tag names are case-insensitive: it doesn't matter whether you write <table> or <Table> or <TABLE>, the parser will go to the table-parsing state regardless.

Tag names are lowercased: after parsing, if you examine the DOM via JS, you'll find that all the tag names are lowercased if the document is HTML, regardless of how you originally spelled it. If you write <Table> in your document, then el.tagName will return "table". In XML, this lowercasing doesn't happen; you'd get "Table" back.

[UnVuTWFu]

Thanks,maybe we clould close this issue