In fact, there was an attempt to add #SMIL support in #HTML: it was called #TIME (Timed Interactive Multimedia Extension)
https://en.wikipedia.org/wiki/HTML%2BTIME
proposed by #Microsoft and #Macromedia and, after being submitted to the #W3C, evolved into the W3C Note (not even a recommendation) for XHTML+SMIL
https://en.wikipedia.org/wiki/XHTML%2BSMIL
No other browser than #InternetExplorer ever added support for it, and honestly, I see that as a loss.

#SVG also supports and incredibly powerful element: #switch. The combination of switch and #SMIL allows some impressively sophisticated things do be achieved in SVG without using #JavaScript or serverside funkiness, and honestly, I love these features SO MUCH that I really wish HTML was extended to support them too.

I love the #SVG format. It's not perfect, but it has some amazing features, and with all the issues in its support across different browsers, it still remains a solid vector graphics format.

One of the things I love the most about #SVG is that it allows interaction and dynamic content without requiring #JavaScript. This isn't actually an #SVG feature _per se_, but it's related to the spec integrating support for #SMIL, and XML language for dynamic content.