Meta: improve PR Preview

[annevk]

This makes PR Preview pages more useful at the expense of making modifications to these files locally a bit more involved.

---

/acknowledgements.html ( diff )
/browsers.html ( diff )
/browsing-the-web.html ( diff )
/canvas.html ( diff )
/common-dom-interfaces.html ( diff )
/common-microsyntaxes.html ( diff )
/comms.html ( diff )
/custom-elements.html ( diff )
/dnd.html ( diff )
/document-lifecycle.html ( diff )
/document-sequences.html ( diff )
/dom.html ( diff )
/dynamic-markup-insertion.html ( diff )
/edits.html ( diff )
/embedded-content-other.html ( diff )
/embedded-content.html ( diff )
/form-control-infrastructure.html ( diff )
/form-elements.html ( diff )
/forms.html ( diff )
/grouping-content.html ( diff )
/iana.html ( diff )
/iframe-embed-object.html ( diff )
/image-maps.html ( diff )
/imagebitmap-and-animations.html ( diff )
/images.html ( diff )
/index.html ( diff )
/indices.html ( diff )
/infrastructure.html ( diff )
/input.html ( diff )
/interaction.html ( diff )
/interactive-elements.html ( diff )
/introduction.html ( diff )
/links.html ( diff )
/media.html ( diff )
/microdata.html ( diff )
/named-characters.html ( diff )
/nav-history-apis.html ( diff )
/obsolete.html ( diff )
/parsing.html ( diff )
/popover.html ( diff )
/references.html ( diff )
/rendering.html ( diff )
/scripting.html ( diff )
/sections.html ( diff )
/semantics-other.html ( diff )
/semantics.html ( diff )
/server-sent-events.html ( diff )
/speculative-loading.html ( diff )
/structured-data.html ( diff )
/syntax.html ( diff )
/system-state.html ( diff )
/tables.html ( diff )
/text-level-semantics.html ( diff )
/timers-and-user-prompts.html ( diff )
/urls-and-fetching.html ( diff )
/web-messaging.html ( diff )
/webappapis.html ( diff )
/webstorage.html ( diff )
/workers.html ( diff )
/worklets.html ( diff )
/xhtml.html ( diff )