v5.3.1

A script nothing waits for no longer holds up the page.

async for meta.scripts

• { "src": "…", "async": true } loads a library with <script async>
  instead of defer. Deferred scripts run in order, and every
  meta.scripts entry is linked ahead of the compiled code — so a large
  library nothing on the page uses, such as Google's 175 KB gtag.js, held
  the page's code, and the first full paint, until it had arrived. On a
  slow phone that was over a second of largest-contentful-paint. An async
  script runs whenever it arrives, as the vendors' own snippets load it.
• A module, or a script whose globals .wf code calls, cannot be async
  — the one is imported by the page's loader, the other might arrive after
  the code that calls it — and is E111.
• The security chapter's tag-manager example loads gtag.js this way.
• For the Rust API: ScriptSpec gains a public is_async field, so code
  that builds one with a struct literal adds is_async: false.

Full Changelog: v5.3.0...v5.3.1