Don’t generate invalid <link rel="canonical"/> with missing href

[andersk]

Remove these invalid HTML tags from the output when context.site is unset:

• <link rel="canonical"/> with missing href
• <meta property="og:url"/> with missing content

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[trueberryless]

Thank you very much for this and all the other awesome PRs.

These changes look good to me. While I couldn't find a resource stating that those tags are invalid without the content attribute, the semantic meaning of those tags is empty.

Here are the resources I found that only say how the tag is intended:

• https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/meta/name
• https://ogp.me/
• https://www.w3.org/TR/rdfa-primer/#using-the-content-attribute

  While there is no problem using link in RDFa Lite (which uses the href attribute, i.e., can be used to define "flavored" links), the fact that, in a conforming HTML file, the meta element may have no text content means that the only way of using the header for such statements is to use the content attribute.

[andersk]

Adjusted as suggested, thanks.

The specification for <link> href says:

One or both of the href or imagesrcset attributes must be present.

For <meta> content, since OpenGraph is based on RDFa, here’s the relevant specification:

If the RDFa @property attribute is present on the meta element, neither the @name, @http-equiv, nor @charset attributes are required and the @content attribute MUST be specified.

[delucis]

It would be good to write some tests for this change to make sure we don’t accidentally regress. These can probably go in packages/starlight/__tests__/head/head.test.ts — we might need to update the getRouteDataTestContext and getTestHead helpers to allow for testing with site not set.