When a static site stops being enough

When you need to keep a secret or write shared state. Everything else, including search over your own content, can be done in the browser with a prebuilt index, and it stays working long after any server you would have written stops being maintained.

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.