HAP is a message-based protocol for retrieving rendered <head> metadata
from single-page applications (SPAs) embedded in iframes. It uses postMessage
and MessageChannel to enable secure metadata transfer from a sandboxed SPA to
a parent document.
HAP defines how a parent document can request the final <head> content and
resolved location.href from an iframe-hosted SPA after JavaScript rendering.
Many SPAs inject their metadata (like <title> and <meta> tags) dynamically
via JavaScript. This protocol provides a reliable way to extract that data once
it's fully rendered, without DOM access or cross-origin issues.
- The parent creates a sandboxed iframe and sends a versioned message:
"HAP:request:1"
- The iframe listens for this message and responds with:
head: HTML string of the<head>tagurl: Finallocation.href
Communication is done via a MessageChannel, which provides a clean, one-to-one
messaging pipe.
"HAP:request:1"(parent → iframe){ head, url }(iframe → parent, viaMessagePort)
See HAP-1 for complete details and example implementations.
- HAP-1: Initial protocol for metadata transfer
Public domain.