This should work when you have the button in focus and press space on keyboard

dabblet.css

/* This should work when you have the button in focus and press space on keyboard  */
/* See https://html.spec.whatwg.org/multipage/scripting.html#selector-active */
/* Firefox currently won't behave properly there! */
button:active {
	background: lime;
}

dabblet.html

<button type="button">Press me using keyboard in Fx</button>

dabblet.js

// alert('Hello world!');

settings.json

{"view":"split","fontsize":"100","seethrough":"","prefixfree":"1","page":"html"}