Explain "invalid at computed-value time" (IACVT)

I've seen the concept of "invalid at computed-value time" (IACVT) crop up on a few content pages now. We ought to write an explainer page for it that we can link to, and not repeat explanations across the site.

Bramus wrote a pretty decent explanation of it here: https://www.bram.us/2024/02/26/css-what-is-iacvt/.