In Firefox I seem to have this https://addons.mozilla.org/en-US/firefox/collections/mozilla/webdeveloper/ installed, and browsing my vanilla MediaWiki site, its console showed me some errors...
GET http://transgender-taiwan.org/index.php?title=%E9%A6%96%E9%A0%81 [HTTP/1.1 200 OK 1573ms] GET http://transgender-taiwan.org/load.php?debug=false&lang=zh-tw&module... [HTTP/1.1 304 Not Modified 569ms] Unknown property 'zoom'. Declaration dropped. @ http://transgender-taiwan.org/load.php?debug=false&lang=zh-tw&module... Expected declaration but found '*'. Skipped to next declaration. @ http://transgender-taiwan.org/load.php?debug=false&lang=zh-tw&module... Expected 'important' but found 'ie'. Expected ';' or '}' to terminate declaration but found 'ie'. Declaration dropped. @ http://transgender-taiwan.org/load.php?debug=false&lang=zh-tw&module... Error in parsing value for 'list-style-type'. Declaration dropped. @ http://transgender-taiwan.org/load.php?debug=false&lang=zh-tw&module... Unknown pseudo-class or pseudo-element '-webkit-input-placeholder'. Ruleset ignored due to bad selector. @ http://transgender-taiwan.org/load.php?debug=false&lang=zh-tw&module...