Html Parser: `Invalid css selector syntax` exception thrown for css pseudo-classes

[EverybodyKurts]

I'm trying to select the first paragraph in a group of sibling paragraphs. According to the MDN nth-child() entry, I should be able to do this using :nth-child().

However, this results in a syntax error when using HtmlDocument.CssSelect:

> node.CssSelect("p:nth-child(1)");;
System.Exception: Invalid css selector syntax (char ':' at offset 0)
   at FSharp.Data.HtmlCssSelectors.tokenize'@104(CssSelectorTokenizer this, FSharpList`1 acc, FSharpList`1 sourceChars)
   at FSharp.Data.HtmlCssSelectors.CssSelectorTokenizer.tokenize()
   at FSharp.Data.HtmlCssSelectors.CssSelectorTokenizer.Tokenize(String pCssSelector)
   at FSharp.Data.HtmlNodeModule.Select(IEnumerable`1 nodes, String selector)
   at <StartupCode$FSI_0029>.$FSI_0029.main@()

[EverybodyKurts]

I see that in http://fsprojects.github.io/FSharp.Data/library/HtmlCssSelectors.html#Implemented-and-missing-features it's listed as a todo. Perhaps instead of a general System.Exception being thrown, a NotImplementedException is thrown?

[EverybodyKurts]

For future reference (notably me), the tokenizing process happens in this function:

https://github.com/fsprojects/FSharp.Data/blob/main/src/Html/HtmlCssSelectors.fs#L103

[github-actions]

🤖 This is an automated response from Repo Assist.

A draft PR has been raised that fixes this: it adds a specific case for unrecognised pseudo-classes/pseudo-elements in the CSS tokenizer so that instead of the misleading "Invalid css selector syntax (char ':' at offset 0)", you now get a NotSupportedException with a message clearly identifying which pseudo-class is not supported and pointing to the documentation listing supported selectors.

Before:

System.Exception: Invalid css selector syntax (char ':' at offset 0)
```

**After:**
```
System.NotSupportedException: CSS pseudo-class or pseudo-element ':nth-child' is not supported. See (fsprojects.github.io/redacted) for the list of supported selectors.

The fix is small (one new match case in HtmlCssSelectors.fs) — all 30 existing CSS selector tests continue to pass.

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