MatFormField should document error accessibility best practices

Right now the only difference between text shown in <mat-hint> and in <mat-error> is the color. This is an accessibility issue because it may not be clear that a message being shown is marked as an error message.

I think that adding a marker directly to <mat-error> would be too opinionated for Angular Material, but we should document a best practice saying that applications should add some sort of marker, such as text or an icon, indicating that the contents of <mat-error> represent an error message.

[jelbourn]

Precisely, yes.

[angular-automatic-lock-bot[bot]]

This issue has been automatically locked due to inactivity. Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}