Add Support for Standard Notes

I spoke with @KraXen72, who uses [Standard Notes](https://standardnotes.com/), an online text editor.

Apparently, it is not well supported by our Chrome extension, partly because it uses a `` element and there is some weird CSS there.

We need to get good support for our Chrome extension on this site.

No files or tests are named. Start by locating the Chrome extension's handling of textarea elements and reproduce the reported behavior in Standard Notes, including its unusual CSS; done means the extension works reliably there without regressions in existing editor support.

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