> at least not anything that's been commonly supported for years or has made it into a standard.
It's more: has made it into a standard and was commonly supported.
Sadly, the browsers aren't trying hard enough not to break anything. They are trying hard not to break anything standard, but the problem with that is that the standards can change, or that some things can be claimed to never have been standards all along. A bunch of IE/Netscape things have broken already such as BLINK and MARQUEE, despite being common enough to "feel" standard even though yes they were never actually standards. Also, as we've seen with the MathML battle in recent years, even standards aren't guaranteed to be kept in browsers if not "commonly supported".
The MDN deprecation warning on FRAMESET:
> Deprecated: This feature is no longer recommended. Though some browsers might still support it, it may have already been removed from the relevant web standards, may be in the process of being dropped, or may only be kept for compatibility purposes.
The browser support table says that every browser still currently supports FRAMESET with simple COLS and ROWS, but as I recall FRAMESET used to also support more complicated layout tools. My brain recalls it as being very similar to TABLE layout at one point in time that you could also have (limited) COLSPAN and ROWSPAN options. Such things may also have been IE/Netscape era "non-standards". If I had examples, they are probably lost to time. Similarly, with nearly no way to easily search in today's indexes for things specifically from the 1990s I can't think of a good way to find old examples either. It's also possibly my memory is just failing me on this and the crazy things I recall doing with framesets were just table layouts after all and maybe iframes, but I do recall doing some crazy things in the 90s that certainly aren't "standard" today and I know wouldn't work in today's framesets.