Step 1 of the spec is to capture the <script> element's node document
into a local variable.
When I originally implemented this, I thought this was not necessary.
However, I realised that the script that runs can adopt the current
script element into a different document, meaning step 5.4 and 6 then
operate on the incorrect document.
Covered by this WPT: 7b0ebaccc6/html/semantics/scripting-1/the-script-element/moving-between-documents-during-evaluation.html
