diff --git a/develop/.htmltest.yml b/develop/.htmltest.yml
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+# Configuration for htmltest, which we run in CI to check that links aren't broken in the built documentation.
+# See all config options: https://github.com/wjdp/htmltest#wrench-configuration
+
+# Don't check external links, as that requires network access and is slow.
+CheckExternal: false
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diff --git a/develop/404.html b/develop/404.html
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