feat: ignore href when hydrating

[Rich-Harris]

Putting this up for discussion. Currently, a SvelteKit app will often render HTML with relative paths, which are replaced with absolute paths upon hydration. (This behaviour is intentional — it means that the HTML is more portable and resilient, for example if it ends up on the Internet Archive or on IPFS. It will likely be the default for all base-relative paths in SvelteKit 2.0.)

The cost is that something like <a href="{paths.base}/whatever"> will differ between SSR and CSR:

• <a href="../whatever">
• <a href="https://example.com/foo/bar/whatever">

As a result, Svelte updates the attribute during hydration, even though it resolves to the same URL.

For src and srcset attributes, we always skip hydration, on the assumption that the resolved value will be unchanged. (In dev, we check that this is in fact the case.) We could do the same thing for href. The stakes are lower — whereas updating src can cause unnecessary network requests, or cause iframes to be reloaded, this is only about avoiding unnecessary work — but it's probably worthwhile anyway.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
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• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

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[dummdidumm]

Agree with the reasoning 👍

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