implementing step 12 of 'prepare a script' algorithm

[psdh]

#4176

[highfive]

Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @metajack (or someone else) soon.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3936

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[Ms2ger]

@psdh: looks pretty good; I left some comments on critic.

[psdh]

Sorry for the delay on this one, it just slipped out of my mind
can someone please help me correct https://critic.hoppipolla.co.uk/showcomment?chain=10458

[jdm]

@psdh The code linked there does not appear to be part of the algorithm described in the original issue, right?

[Ms2ger]

@psdh: another passing test!

/html/semantics/scripting-1/the-script-element/script-for-onload.html
---------------------------------------------------------------------
PASS expected FAIL Script for and onload attributes