Tracking issue for lints for the `html!` macro.

Describe the feature you'd like
The html! macro should output warnings. The developer should be able to ignore these warnings as they see fit.

These warnings should integrate into the compiler and work just as any other lint/warning would.

Is your feature request related to a problem? Please describe. (Optional)
Often it's easy to make mistakes based on

Describe alternatives you've considered (Optional)
A clear and concise description of any alternative solutions or features you've considered.

Additional context (Optional)
This is blocked on the diagnostics API of rustc becoming available in a stable release of Rust.

Implemented lints include:

• Checking that all <a> tags have a href attribute (this is to enable screen readers to work on websites).
• Checking that all <img> tags have an alt attribute (this is to enable screen readers to work on websites).

Possible lints include:

• ...

Possible sources of inspiration for lints include:

• These two pages from the W3C: this page about getting started with web accessibility and this page about more detailed web accessibility concerns.

Please feel free to suggest lint ideas below, or further updates on the status of the diagnostics API.

The entry point is the html! macro; begin by reviewing the existing lint items and the linked rustc diagnostics API dependency. Done means the remaining lint scope is defined and warnings integrate with the Rust compiler while remaining suppressible, with accessibility guidance from the linked W3C pages.