A video's file says its size only once it starts loading. Until then the page holds a 16:9 box for it, and when the video is another shape, the text below it moves. Write the video's width and height in pixels, so the page keeps its room from the start. media({ size: true }) reads them from remote files for trusted documents, and a data URI is measured without it. A GitHub attachment link takes no props, so it isn't reported.
Incorrect
Correct
{width=1280 height=720}