There are already webcams which do this- but they use a mime trick for 'multipart/x-mixed-replace'.

That's basically the server telling the client 'That data I just sent you, well now replace it with this new thing'.

No JavaScript needed, and can work with plain http and jpeg

A lot of IP cameras already do this via https://en.wikipedia.org/wiki/Motion_JPEG

You can do this with the gif too, I have once created a toy cgi that combined a gif sending one frame at a time with an image map allowing you to Remote Desktop with no JavaScript and click around. JPEG may have been a better choice, maybe I’ll revisit this.