[WIP] Finish implementation of HTML5 form validation.

[gpoesia]

This is still work in progress, but comments would be highly appreciated.

Some questions:

1- The standard doesn't mention date and telephone validation (I checked which sections refer to the "type mismatch" validity state, and only URL and Email were there). That's why I left it out, but the last PR (#10843) had some validation for them. Should I bring it back?

2- In URL validation, I've checked whether there is either a '.' or a '/' (since for instance http://localhost is a valid URL). Is that fine?

3- For emails, I've only checked whether there is an '@' in the text. However, there's a suggested regex in the standard. Should we use it?

4- In the 'pattern mismatch' error, how do I use JavaScript's Regex engine? I'm guessing the regex crate might differ.

5- How do I handle 'multiple' inputs? Since there's only one value in an input element, I'm not sure how multiple inputs should be validated.

6- What does /tests/html/form_html5_validations.html actually test? Or is it simply a page for manual testing?

TODO:

• Check for missing validity flags in HTMLInputElement: range errors (underflow/overflow), step mismatch, bad input, custom error
• Implement HTMLSelectElement::validate
• Implement HTMLTextAreaElement::validate
• Implement the constraint validation API
• Write WPT tests
• Anything else?

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• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #11444 (github issue number if applicable).

• There are tests for these changes OR
• These changes do not require tests because _____

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This change is 

[highfive]

Heads up! This PR modifies the following files:

• @fitzgen: components/script/dom/htmlformelement.rs, components/script/dom/htmlinputelement.rs, components/script/dom/validitystate.rs
• @KiChjang: components/script/dom/htmlformelement.rs, components/script/dom/htmlinputelement.rs, components/script/dom/validitystate.rs

[highfive]

Warning

• These commits modify script code, but no tests are modified. Please consider adding a test!

[bors-servo]

☔ The latest upstream changes (presumably #17822) made this pull request unmergeable. Please resolve the merge conflicts.

[KiChjang]

This is just a prelimiary review and I'm only highlighting things that really jumped out to me.

The most surprising part here is the validation logic. I had expected that most of the validation logic to be self-contained in validitystate.rs instead of being shoved all into the validate method in htmlinputelement.rs.

[KiChjang]

The _ prefix can be removed from validate_flags.

[KiChjang]

Why is this call to borrow necessary?

[KiChjang]

@gpoesia Are you still working on this?

[bors-servo]

☔ The latest upstream changes (presumably #18262) made this pull request unmergeable. Please resolve the merge conflicts.

[KiChjang]

Closing due to inactivity.