First-party vs third-party stylesheets on React sites

Sites with first-party stylesheets stylesheet origin (first-party vs third-party) have a good LCP most often: 85%, against 85% of all React sites. Sites with third-party stylesheets CSS origin have it least often, at 78%.

First-party stylesheets is the most used: 99.9% of React sites.

Across the four LCP sub-parts in Chrome's field data, the largest gap between first-party stylesheets and third-party stylesheets sits in time to first byte (TTFB): 516 ms median for first-party stylesheets against 589 ms for third-party stylesheets. Overall median field LCP is 1.4s for first-party stylesheets (TTFB 516 ms, load delay 439 ms, load duration 146 ms, render delay 175 ms) versus 1.6s for third-party stylesheets.

Wix sets the stylesheet origin (first-party vs third-party) to first-party stylesheets on 100% of its sites, and 94% of Wix sites have a good LCP, against 85% of all sites.

PlatformStylesheet origin (first-party vs third-party) on its sitesGood LCP

React sites with stylesheet origin (first-party vs third-party) 9,569 sitesFirst-party stylesheets 100%Third-party stylesheets 45%85%

Wix 5,824 sitesFirst-party stylesheets 100%Third-party stylesheets 34%94%

WordPress 2,342 sitesFirst-party stylesheets 100%Third-party stylesheets 62%61%

First-party stylesheets Third-party stylesheets

Does the platform explain the stylesheet origin (first-party vs third-party) difference?

On Wix, 94% of sites with first-party stylesheets stylesheet origin (first-party vs third-party) have a good LCP, against 93% of Wix sites with third-party stylesheets. Within Wix, sites with first-party stylesheets and third-party stylesheets have a good LCP about equally often. On WordPress, 61% of sites with first-party stylesheets stylesheet origin (first-party vs third-party) have a good LCP, against 56% of WordPress sites with third-party stylesheets. Within WordPress, sites with first-party stylesheets have a good LCP more often.

How cross-origin CSS files add DNS and TLS round-trips to the critical rendering path

Because external stylesheets block rendering by default, fetching a CSS file from a third-party domain (such as fonts.googleapis.com, cdnjs, or a widget CDN) stalls first paint behind a fresh DNS lookup, TCP handshake, and TLS negotiation. Across 9,557 sites (99.9%) with first-party CSS, good LCP is 85% (1.4s median), whereas across 4,282 sites (44.7%) with third-party CSS, good LCP drops to 78% (1.6s median).