Document ineffectiveness of some common <input> attributes for date

[andypillip]

I noticed that placeholder was valid, but wouldn’t be applied on , at least in Firefox.

Summary

Mention ineffectiveness of some common input attributes for date.

Motivation

I had a doubt placeholder attributes were effective on , so I read in the attributes section, which didn’t mention that at all.

Supporting details

Related issues

Metadata

• [ ] Adds a new document
• [x] Rewrites (or significantly expands) a document
• [ ] Fixes a typo, bug, or other error

[sideshowbarker]

Andy, thanks much, and congrats on landing your first docs change here — welcome aboard 🎉