What is most useful to send
A tool that gave a wrong answer. This is the most valuable message this address receives. Include what you pasted, what the tool said, and what the real system did instead. Every tool here states what it cannot see, and if one of those statements is itself wrong, that is worth knowing before anyone else relies on it.
A tool that is missing. The best tools here exist because somebody described a problem that kept costing them an afternoon. The useful version is the problem rather than the proposed solution.
A correction to an explanation. Every page carries prose about how something actually behaves, drawn from specifications and upstream sources. If a claim is out of date, or was never right, please say so and point at what it should be.
Why there is no form
A contact form has to send what you type somewhere, and this site has no server to send it to: it is static files and nothing else, which is the same property that lets every tool promise your data never leaves the tab. Using a third-party form service would put a script from another origin on the page, which is precisely the thing that promise rules out.
An email address is less convenient and keeps both promises intact, so that is the trade taken here.
What not to send
Please redact credentials, tokens, private keys and internal hostnames before sending a sample. A reduced example that still reproduces the problem is more useful than a real file anyway, and email is not a secure channel.
Response times
This is maintained alongside other work, so replies are not immediate and there is no support commitment. A reproducible wrong answer gets looked at first.