This error is most likely being caused by your inclusion of the HTML 5 tags and in your code. Can you tell me what version of CM1 you are running? More robust support for HTML 5 tags within the Rich Text widget will be delivered in our next major release, CM1 3.0 (which we plan to make public early next week), including support for and tags. I have actually just tested your code using our latest internal build of 3.0, and the tags didn’t cause errors, nor were they stripped / replaced.
Dan, that makes sense. I didn’t have a 2.11 environment that I could quickly spin up to test this, but I recalled encountering this type of error message in a recent release (2.13 simply replaces these HTML 5 tags, rather than throw the error message you’re seeing).