Sites like github.dev and Codespaces allow you to use the VS Code extensions you love, even in the browser.
From the documentation:
Web extensions share the same structure as regular extensions, but given the different runtime, don't run with the same code as extensions written for a Node.js runtime. Web extensions still have access to the full VS Code API, but no longer to the Node.js APIs and module loading. Instead, web extensions are restricted by the browser sandbox and therefore have limitations compared to normal extensions.
If you're an extension developer, I encourage you to give the Web Extensions documentation a read. There's all kinds of stuff in there including how to test a web extension.
Happy VS Coding!