[fix] HTML crawl regression

[Conduitry]

Whoops. #3668 introduced a bunch of huge bugs because substr is not the same as substring. We could keep using substr even though it's deprecated, or we could rewrite this. I've opted for the former because I want to get this fixed quickly.

There are also no new tests. Ideally this dumb bug would have been caught by a test, but it wasn't, and I'm not going to write more right now, also because I want to get this fixed quickly.

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