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Mostly anything that relies on having a real system attached to it instead of the browser JS engine. Running code, or a terminal, or things that rely on the git binary. Lots of extensions are working to remove their dependencies on the local system though.

So these limitations don't exist for Codespaces then?

They do not.

If you were to continue reading, you would be informed that many of the non-web languages don't have the same support in this version, since it can't run native language servers/any extensions that use OS-level APIs.

Some extensions aren't available, such as GitLens.

Yeah, the first thing I tried was to install some Go extensions. I guess they'll need to have separate web versions (or some sort of extra qualifiers)