Web font swaps cause a visible reflow. Fix

font-display: swap plus a fallback stack with matched metrics. The modern lever is size-adjust and ascent-override on the fallback @font-face, which lets the substitute occupy the same space, so the swap becomes invisible rather than merely fast.

Because the browser does not know their size until the bytes arrive. Set width and height attributes, or an aspect-ratio in CSS, and the space is reserved from the first layout pass.

An afternoon to do it properly the first time, then minutes when it recurs. Most of the first afternoon goes into discovering which of the defaults do not match your situation.