Implement encoding determination for external scripts.

[Ms2ger]

• Spec: https://html.spec.whatwg.org/multipage/scripting.html#establish-script-block-source.
• Code: components/script/dom/htmlscriptelement.rs, fn execute.
• Tests: none that I know of, so I'll be expecting quite a few tests to be written as part of this issue. These should probably live in (a subdir of) tests/wpt/web-platform-tests/html/semantics/scripting-1/the-script-element.

[njskalski]

I attempted to do something here, but I am not sure if I get steps 3 and 4 right.

master...askalski:issue9185

[njskalski]

I was unable to locate "script block's fallback character encoding", and I don't know which one is "character encoding"

should any of these mimic this:
https://developer.mozilla.org/en-US/docs/Web/Guide/Localizations_and_character_encodings#Specifying_the_fallback_encoding
?

[njskalski]

From what I understand fromhttps://html.spec.whatwg.org/multipage/scripting.html#establish-script-block-source , I should deduce "script block's fallback character encoding" from document's one.

I added two "HELPME" comments, with specific questions. I would appreciate some suggestions.

[Ms2ger]

First, in Servo right now, "the script block's fallback character encoding" will always be UTF-8, so feel free to read "UTF-8" where that is written.

Second, I think we'll need to change the type of block_character_encoding. Rather than having it be DOMRefCell<EncodingRef> and initializing to DOMRefCell::new(UTF_8 as EncodingRef), I think we should make it a DOMRefCell<Option<EncodingRef>> and initialize it to DOMRefCell::new(None). This will allow you to implement the "If the algorithm above set the script block's character encoding" check.

[njskalski]

I implemented suggested changes. I open PR now to see Travis CI output #9248

I also have a question what tests should look like.
Should I create wpt tests with simple pages, in which I have assertions that loaded external js files have some property set? If so, I need a suggestion. I have never used charset properties from JS.

Or should I write some unit tests in Rust?

[jdm]

Fixed by #10079.