HTTP/2 Server Push or 103 Early Hints configured

HTTP/2 Server Push and 103 Early Hints both address the same problem: the browser cannot start fetching critical assets (CSS, fonts, JS) until it has received and begun parsing the HTML response. Early Hints tells the browser what to fetch before the HTML is ready — shaving 50–200ms from FCP and LCP on cold loads. ISO 25010:2011 time-behaviour captures the gain; for users on high-latency connections (mobile networks), this optimization is especially valuable because each round-trip is expensive.

Configure 103 Early Hints for your critical CSS and font assets via deployment headers. On Vercel, this is handled automatically; for custom servers, emit the 103 response before awaiting the full render.

// vercel.json — early hints for font and CSS
{
  "headers": [
    {
      "source": "/",
      "headers": [
        { "key": "Link", "value": "</fonts/inter.woff2>; rel=preload; as=font; crossorigin, </styles/main.css>; rel=preload; as=style" }
      ]
    }
  ]
}