Invalid / non-standard HTML output

I noticed, while working on #2861387: Use a Twig template for generating html output, that the output of the module does not pass HTML5 validation. The HTML output of the module triggered a couple of errors:

• Attribute transparency not allowed on element iframe at this point.
• The frameborder attribute on the iframe element is obsolete. Use CSS instead.
• The scrolling attribute on the iframe element is obsolete. Use CSS instead.
• Text not allowed in element iframe in this context.

Note that not all documentation sources agree with these errors. For instance, MDN says fallback content is allowed while the W3 standard says no.

I guess it's time to discuss if and how strict we want to adhere to the W3 standard.