Definition of valid shadow elements is ambiguous

[MatsPalmgren]

https://dom.spec.whatwg.org/#dom-element-attachshadow
Item 2 in the list says:

If context object’s local name is not a valid custom element name, "article", "aside", "blockquote", "body", "div", "footer", "h1", "h2", "h3", "h4", "h5", "h6", "header", "main" "nav", "p", "section", or "span", then throw a "NotSupportedError" DOMException.

The above sentence is ambiguous. The enumeration can be read both as a list of valid names, or as a list of invalid names. The sentence is also unnecessarily complex. I would suggest:

If context object’s local name is not a valid custom element name, then throw a "NotSupportedError" DOMException. The only valid names are: "article", "aside", "blockquote", "body", "div", "footer", "h1", "h2", "h3", "h4", "h5", "h6", "header", "main" "nav", "p", "section", and "span".

[rniwa]

That doesn't read well. It sounds like we should always throw NotSupportedError whenever the context object's local name is not a valid custom name. We need something like this:

If context object’s local name is not one of a valid custom element name, "article", "aside", "blockquote", "body", "div", "footer", "h1", "h2", "h3", "h4", "h5", "h6", "header", "main" "nav", "p", "section", or "span", then throw a "NotSupportedError" DOMException.

or

If context object’s local name is not a valid custom element name and not one of "article", "aside", "blockquote", "body", "div", "footer", "h1", "h2", "h3", "h4", "h5", "h6", "header", "main" "nav", "p", "section", or "span", then throw a "NotSupportedError" DOMException.

[annevk]

I'd also support a list:

If context object’s local name is not one of

• a valid custom element name
• "article"
• "aside"
• ...

then throw ...

(Note that the rewording in OP is incorrect.)

[MatsPalmgren]

Oh, so "a valid custom element name" was intended to be one of the alternatives in that enumeration? Then it's even worse than I thought, because that meaning didn't even occur to me. I might have understood it like that if there had been a colon after the "not", maybe. @rniwa's first suggestion have that same problem IMO. The second suggestion is better but still rather convoluted. Perhaps making an explicit listing of alternatives is better:

Context object’s local name must be one of the following:

• a valid custom element name
• "article", "aside", "blockquote", "body", "div", "footer", "h1", "h2", "h3", "h4", "h5", "h6", "header", "main" "nav", "p", "section", or "span"

If it's not, then throw a "NotSupportedError" DOMException.

[Yash-Handa]

Hey @annevk, I would love to help in editing the dom.bs.

BTW, for clarification the following should be done:

this’s local name must be one of the following:

• a valid custom element name.
• "article", "aside", "blockquote", "body", "div", "footer", "h1", "h2", "h3", "h4", "h5", "h6", "header", "main", "nav", "p", "section", or "span".

If it's not, then throw a "NotSupportedError" DOMException.

A suggestion though (a less verbose option):

If this’s local name is not an "article", "aside", "blockquote", "body", "div", "footer", "h1", "h2", "h3", "h4", "h5", "h6", "header", "main", "nav", "p", "section", "span", or any valid custom element name, then throw a "NotSupportedError" DOMException.

Do let me know which one should be used : )

[annevk]

Thanks @Yash-Handa!

I'd suggest:

If this's local name is not one of the following:

• ...
• ...

then throw a ...