I opened the STP 247 notes on my lunch break and there is one headline, plus a scatter of small fixes that a lot of readers will feel without ever reading a changelog. The headline is CSS calc-mix() landing in WebKit's preview channel. The scatter is a couple of HTML spec-alignment repairs, two accessibility fixes, and a MathML tweak.
This is Safari Technology Preview 247, dated July 1, 2026, covering WebKit changes between 314716@main and 315846@main. It is a preview build, not a stable Safari release, so treat everything here as engine-specific behaviour you can experiment with today rather than something to ship broadly tomorrow.
The one CSS line worth pinning
The release lists "Added support for calc-mix()" under CSS → New Features, and that is the whole entry. No syntax example, no follow-up prose. So the useful thing I can say from the notes alone is that WebKit's preview engine now knows the function name. That is the point where a hand-authored stylesheet can start feature-detecting it in this build and dropping it into a scratch page.
Because the release does not spell out the syntax, I am not going to paste one here. Read the linked spec text before you write more than a scratch rule against it, and gate anything real behind an @supports block until you know what other engines have.
