Emulate print media in Firefox 62 (userscript work in progress)

showAsPrint.js

// For Firefox's Web Console, creates the functions showAsPrint() and undoShowAsPrint()
// to roughly emulate print media and revert
function showAsPrint(){
	var docSS = document.styleSheets, ss, oldMedia, newMedia, rules;
	var p2s = function(media){
		if (media.indexOf('all') > -1) return media; //no need to change
		if (media == 'print') return 'all, wasprint'; //show on screen, too
		if (media.indexOf('print') > -1 && media.indexOf('screen') > -1) return media; //no need to change
		if (media == 'screen') return 'wasscreen'; //hide these rules
		if (media.indexOf('screen') > -1) return media.replace('screen', 'wasscreen'); //hide these rules
		if (media.trim().slice(0,1) == '(' && media.trim().slice(-1) == ')') return media; //too hard to parse
		return media + ', WTF'; //for debugging only
	}
	for (var i=0; i<docSS.length; i++) {
		ss = docSS[i];
		if (!ss.disabled){
			if (ss.ownerNode && ss.ownerNode.id.indexOf('stylus-') == -1 && ss.ownerNode.id.indexOf('stylish-') == -1){
				// check link or style tag for media attribute and edit as needed
				if (ss.ownerNode.hasAttribute('media') && ss.ownerNode.getAttribute('media') != 'all'){
					oldMedia = ss.ownerNode.getAttribute('media');
					newMedia = p2s(oldMedia);
					if (newMedia != oldMedia){
						//console.log('Updating: ' + ss.ownerNode.outerHTML + ' to ' + newMedia);
						ss.ownerNode.setAttribute('media', newMedia);
					}
				}
				// check content of style sheet for media rules
				try { 
					rules = ss.cssRules; 
				} catch(err) { 
					console.log('Can\'t access cssRules for ' + ss.href + ' due to: ' + err ); 
					rules = []; 
				}
				for (var j=0; j<rules.length; j++){
					if (rules[j].type == 4){
						oldMedia = rules[j].conditionText;
						newMedia = p2s(oldMedia);
						if (newMedia != oldMedia){
							//console.log('Updating CSSMediaRule from ' + oldMedia + ' to ' + newMedia);
							rules[j].conditionText = newMedia;
						}
					}
				}
			} else if (!ss.ownerNode){
				console.log('No .ownerNode on i=' + i);
			}
		}
	}
}
function undoShowAsPrint(){
	var docSS = document.styleSheets, ss, oldMedia, newMedia, rules;
	var unp2s = function(media){
		if (media == 'all, wasprint' || media == 'all,wasprint') return 'print'; //undo applying to screen
		if (media == 'wasscreen') return 'screen'; //undo hiding
		if (media.indexOf('wasscreen') > -1) return media.replace('wasscreen', 'screen'); //undo hiding
		return media; // otherwise, we didn't mess with it
	}
	for (var i=0; i<docSS.length; i++) {
		ss = docSS[i];
		if (!ss.disabled){
			if (ss.ownerNode && ss.ownerNode.id.indexOf('stylus-') == -1 && ss.ownerNode.id.indexOf('stylish-') == -1){
				// check link or style tag for media attribute and edit as needed
				if (ss.ownerNode.hasAttribute('media') && ss.ownerNode.getAttribute('media') != 'all'){
					oldMedia = ss.ownerNode.getAttribute('media');
					newMedia = unp2s(oldMedia);
					if (newMedia != oldMedia){
						//console.log('Restoring: ' + ss.ownerNode.outerHTML + ' to ' + newMedia);
						ss.ownerNode.setAttribute('media', newMedia);
					}
				} else {
					// check content of style sheet for media rules
					try { 
						rules = ss.cssRules; 
					} catch(err) { 
						console.log('Can\'t access cssRules for ' + ss.href + ' due to: ' + err ); 
						rules = []; 
					}
					for (var j=0; j<rules.length; j++){
						if (rules[j].type == 4){
							oldMedia = rules[j].conditionText;
							newMedia = unp2s(oldMedia);
							if (newMedia != oldMedia){
								//console.log('Restoring CSSMediaRule from ' + oldMedia + ' to ' + newMedia);
								rules[j].conditionText = newMedia;
							}
						}
					}
				}
			} else if (!ss.ownerNode){
				console.log('No .ownerNode on i=' + i);
			}
		}
	}
}

Firefox 70 has a button for this now in the Page Inspector, small document icon on the same bar as "Filter Styles" in the Rules pane.

(2024-03-26, 5 years after last comment)

1. the function doesn't work (chromium 122, firefox 123).
   Calling the function doesn't give any error.
   It's just ends immediately, silently, without any visible change to the page.

(p.s. i was hoping to convert it to a bookmarklet.)