It used to not work for responsive images. We had to do tricks like pseudo elements with padding set to a percent value: https://css-tricks.com/preventing-content-reflow-from-lazy-l...
https://jaeh.at works fine when reloading, no layout changes, and the layout changed slightly before i fixed it using width & height.
https://noncon.org does not (and i had no time to fix, might be because width of images is not set, might be something with the css position/float layout, we had about a week to setup the whole conference infrastructure, including the streaming server, layout flashing was my last concern at the time and i kept myself too busy since then to fix something so trivial...)
sorry for the personal links, just examples i knew from the top of my head because i built them ;)