Input in Memo/Rich Text Editor custom field results in error "There is a validation error with your HTML input"
The error you're getting is from the IDS (Intrusion Detection System).  CiviCRM has a third-party IDS package that's bolted on, and it can certainly be overzealous.  Its purpose is to prevent people from, e.g., submitting spam URLs into public forms.

However, assuming that you're logged in as an authenticated user and having this problem, the easiest solution is to grant yourself the "CiviCRM: skip IDS check" permission.  You haven't specified your CMS, so I can't tell you where your permissions are, but grant your role that permission and you should see the error disappear.