pub fn set_head_html(html: &str)Expand description
Record the head HTML a page rendered, and merge it into the document’s head the way a browser
host does (runtime.js’s mergeIntoHead): each element with an identity replaces every element
of that identity already there, and joins the head at its end; one without is simply added.
Called once, at mount.
What is recorded (head_html) is the page’s declared head after that same merge — a layout’s
Head: and the page’s own arrive as one string, and a server composing the document from it
must see the page’s title, not both — each surviving tag exactly as it was written.