I have some knowledge of HTML4 but I have never used HTML5, I now find I may be able to improve one HTML4 page (which uses Frames) if I could use the "drop down" element available in HTML5 but what I read about HTML5 confuses me.
According to what I read, modern browsers are fully backward-compatible and I can easily change only the first line of the page (the 'doctype' line) to make it an HTML5 document and leave everything else as is. But HTML5 "does not support frames"..so the browsers cannot be "fully backward-compatible".. or will they consider the page as a W3C validator incorrect page but happily process it as a mixture of both??

Of course it's only a matter of trying but I thought it wiser to ask advice first.

Thank you for your help.

i don't think any browser is currently interested in removing support for outdated or badly coded sites. hell, even flash is still supported.

so your frames will likely continue to work.
that said, i think they were an internet explorer thing originally and in 2019 no self-respecting web designer would touch them with a 10-foot selfie stick.

what's a "html5 drop down element"? maybe this? and how is it going to replace a frame?

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“Backward compatible” means old code will continue to be supported. I’m sure there’s more to changing to HTML5 than changing the doctype metatag...Which is not even required to be present.

And I too wonder what the OP means by “html5 drop down element”. As ondoho has said, that term usually describes a type of form element...

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If you like being standards compliant and up-to-date, replace those frames with suitable divs as part of a process of updating the entire web page/site to html5. It will be a useful and educational exercise for you.

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