Adoption of modern HTML and CSS features on Tranco top-600 home pages (August 2026)

A single-run, reproducible measurement of how often eighteen modern web-platform features actually appear in the source served by widely used websites. Sample: first 600 domains of Tranco list W3779 (created 2026-08-09). Collected: 2026-08-10. One HTTPS GET per home page, up to four linked stylesheets plus inline style blocks; detection by literal pattern matching on the served bytes. Denominators: 320 of 600 domains returned HTTP 200 (basis for HTML features); 306 of those exposed at least one readable stylesheet (basis for CSS features). 280 domains are excluded and are not counted as feature-absent: 205 unreachable, 38 refused (403), 16 returned 404, the rest assorted statuses. The top of the Tranco list contains DNS and CDN infrastructure domains that serve no web page. Selected results: CSS custom properties 81.7%, aspect-ratio 51.3%, :has() 50.0%, srcset 46.9%, loading=lazy 42.8%, @container 18.3%, subgrid 10.5%, <dialog> 3.8%, popover 0.9%, importmap 0.6%. Limits: only served bytes are read, so anything injected by JavaScript after hydration is invisible; home pages only; a marker proves declared use, not rendered use; four stylesheets per page; one run on one day, so no trend is claimed. Full method and limits in README.md.