UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.100 Safari/537.36

Steps to reproduce the problem:
1. Go to any website featuring a Form that can be submitted by pressing the [Enter] key.
2. Type in something.
3. Hold down the Enter key.

Any keyboard-initiated form submit will be repeated at the repeat-rate that the keyboard is set to. If your keyboard is set to the highest repeat rate, that's about 20 requests a second.

This only stops if the form you're submitting causes a page reload. In the case of an AJAX-based form which modifies page content, this never happens and you can sit there throwing 20 req/s at the target server.

You can replicate this with https://wordpress.org/search/derp
This form points back to googleapis.com which will eventually rate-limit you with a 403 and tell you to get knotted.

What is the expected behavior?
Ideally the form submit would only be initiated once per press of the Enter key.

For backwards compatibility however, it would be nice if the form Submit event was at least throttled to a sensible rate, and not slave to the OS keyboard repeat-rate.

What went wrong?
Legitimate users are very easily able to:
1. Request flood a server
2. Get themselves blocked for too many requests
3. Accidentally make duplicate submissions.

Did this work before? No 

Chrome version: 61.0.3163.100  Channel: n/a
OS Version: 10.0
Flash Version: 

This appears to be the standard behaviour of ALL current major browsers, although I can't understand why this would be the desired default behaviour for 99% of the web forms out there.

This can be *somewhat* easily worked around with custom JavaScript, but again it doesn't seem desirable.