Limitations & Rules on Content of SP List Item Columns of Type Multiple Lines Of Text

In setting up REST requests to update fields in SP List items whose data type is Multiple Lines of Text of the Enhanced Rich Text option, I am creating markup with anchor elements (links).

I am discovering that I cannot I cannot use anchor elements with the attribute href="" being the empty string, and attribute "onclick" whose value is a function call like "editDocumentWithProgID2()". SharePoint appears to be creating something like "<a>contained text</a>" which basically looks like a link but clicks to nothing.

So perhaps there is documentation on the limitations and rules of contained text and markup within this SP column type somewhere. Where is it?