Document why the border on the character count input doesn't show as an error when the content is under the limit

[christopherthomasdesign]

What

Document why the border on the character count input doesn't show as an error state (i.e. thick red border) when there's been an error but content is under the limit.

For example:

Why

A couple of users have raised issues about this – see https://github.com/alphagov/govuk-frontend/issues/2588 and https://github.com/alphagov/govuk-frontend/issues/1858

Who needs to work on this

Content designer, probs with accessibility specialist or frontend dev

Who needs to review this

Anyone

Done when

• [x] Rationale understood
• [x] Content written

[EoinShaughnessy]

We've agreed wording in the PR.

However, we're holding off on publishing the change until we figure out how to release it alongside the bigger update to the same guidance.