SVG is just html5, it has full support for CSS, javascript with buttons, web workers, arbitrary fetch requests, and so on (obviously not supported by image viewers or allowed by browsers).
Browsers support all that sort of thing, as long as you use an iframe. (Technically there are sone subtle differences between that and html5, but you are right its mostly the same)
If you use an <img> tag, svgs are loaded in "restricted" mode. This disables scripting and external resources. However animation via either SMIL or CSS is still supported.
And non-browser image renders support almost none of those advanced totally-still-SVG features (and I don't blame them) while they often do support animated GIFs.
SVG is just html5, it has full support for CSS, javascript with buttons, web workers, arbitrary fetch requests, and so on (obviously not supported by image viewers or allowed by browsers).