* the actual existence of frames (although those are deprecated)
* iframes (which are not deprecated, so seemingly doing declarative inclusion of HTML in HTML was not what was wrong with frames)
* imports in CSS, which share some of the same problems / concerns as HTML imports
* the existence of JavaScript with its ability to change anything on the page, including the ability to issue HTTP requests and be written arbitrarily obfuscated ways.
* the actual existence of frames (although those are deprecated)
* iframes (which are not deprecated, so seemingly doing declarative inclusion of HTML in HTML was not what was wrong with frames)
* imports in CSS, which share some of the same problems / concerns as HTML imports
* the existence of JavaScript with its ability to change anything on the page, including the ability to issue HTTP requests and be written arbitrarily obfuscated ways.