Interesting. It is using iframes behind the scenes. According to W3C it should limit the recursion depth to one, but it seems that you can get around that.
> The runtime and the tooling go together. The reason why tooling doesn't exist for HTML5, SVG, and Javascript is it's just so damn complex and fiddly that it's impossible to paper-over and abstract with a robust authoring tool.
Indeed. The fact that Flash is proprietary meant that all browsers had to use the same software (up to versioning and OS API implementations) to run SWF files. Now with HTML5, developers now have to deal with differences between different browsers (Chromium/Safari/Firefox are the most popular ones).
Relevant: https://www.bryanbraun.com/2021/03/24/infinitely-nested-ifra...