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This allows accurate line numbers when reporting stylesheet errors.
@jdm This is going to require some effort to merge my changes with other recent changes to `ParserContext`. Because of that I would appreciate a quick sanity check before I put the time into performing the merge.
For example, should I store the `offset` as a u64, or should it be an Option?
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Source-Repo: https://github.com/servo/servo
Source-Revision: 94397ff0b4fed138241760770ff2d230985bed8a