Redirect from prerendered pages with `<script>`

[Rich-Harris]

Describe the problem

When prerendering, if we encounter a redirect it gets baked out as an HTML file with a <meta> refresh tag:

kit/packages/kit/src/core/postbuild/prerender.js

Lines 341 to 344 in bc70b4e

	writeFileSync(
	dest,
	`<meta http-equiv="refresh" content=${escape_html_attr(`0;url=${location}`)}>`
	);

Turns out (h/t @cramforce) it's actually quicker to use a <script>:

<script>onload=() => location.href = "${url}"</script>

(Do we even need the onload? Would it be quicker still without it?)

Describe the proposed solution

Do both — <script> for speed, <meta> for reliability.

Alternatives considered

No response

Importance

nice to have

Additional Information

No response

[cramforce]

The onload is for if you want to guarantee the page gets painted (like for a loading dialog UI or similar). It would only be good if the page has zero sub-resources, otherwise it would lead to unpredictable delays.

[Conduitry]

In this case, we're using a prerendered page to simulate a server side redirect, and the file will contain only the <meta> tag - there are no user-visible elements at all that we have to worry about getting partially displayed. It sounds like we should just add a direct <script>location.href = "..."</script> then.