This works around this Safari bug.
https://bugs.webkit.org/show_bug.cgi?id=290146
This unfortunate because it may cause additional layouts if there's more
updates to the tree coming by manual mutation before it gets painted
naturally. However, we might end up wanting to read layout early anyway.
This affects the fixture because we clone the `<link>` from the `<head>`
which is itself another bug. However, it should be possible to have
`<link>` tags inserted into the new tree so this is still relevant.
e1e740717b