I adore bookmarklets, I use:

javascript:(function(){function%20init(){var%20newline=unescape(%22%%22+%220A%22);dead=false;oldCSS=null;x=opener;ta=document.f.ta;ta.select();ta.value=%22/*%20Type%20CSS%20rules%20here%20and%20they%20will%20be%20applied%20to%22+newline+%22whatever%20page%20is%20loaded%20in%20that%20tab,%20as%20long%20as%20the%20pages%22+newline+%22are%20from%20'%22+location.host+%22'%22+newline+%22and%20you%20keep%20this%20window%20open.%20*/%22+newline+newline;update();}function%20update(){try{if(!x||x.closed){ta.style.backgroundColor=%22#ddd%22;return;}x.bookmarkletStyleSheet;}catch(er){ta.style.backgroundColor=%22#fdc%22;setTimeout(update,150);dead=true;return;}if(dead){dead=false;ta.style.backgroundColor=%22%22;oldCSS=null;}if(!x.testStyles){var%20newSS;newSS=x.document.createElement(%22link%22);newSS.rel=%22stylesheet%22;newSS.type=%22text/css%22;x.document.getElementsByTagName(%22head%22)[0].appendChild(newSS);x.testStyles=newSS;oldCSS=null;}if(oldCSS!=ta.value){oldCSS=ta.value;if(window.opera)x.testStyles.href=%22javascript:unescape('%22+escape(ta.value)+%22')%22;else%20if(navigator.userAgent.indexOf(%22MSIE%22)!=-1)x.testStyles.href=%22javascript:unescape('%22+escape(escape(ta.value))+%22')%22;else%20x.testStyles.href=%22data:text/css,%22+escape(ta.value);}setTimeout(update,150);}y=window.open('','','resizable,width=500,height=300');y.document.write('<title>New%20CSS%20Style%20Sheet</title><style>.ec%20{%20width:%20100%;%20height:%20100%;%20border:%20none;%20margin:%200px;%20padding:%200px;%20}</style><body%20class=%22ec%22><form%20name=%22f%22%20style=%22margin:%200px;%22%20class=%22ec%22><textarea%20name=%22ta%22%20wrap=%22soft%22%20style=%22margin:%200px;%20border:%200px;%20width:100%;%20height:100%;%22%20class=%22ec%22></textarea><script>'+update+init+'init();<'+'/script>');y.document.close();})()

To quickly edit the css style of anything.

And:

javascript:(function()%7Blet%20style%20%3D%20document.getElementById('__colorallblocks__')%3Bif%20(style)%7Bstyle.remove()%3B%7D%20else%20%7Bstyle%20%3D%20document.createElement('style')%3Bstyle.id%20%3D%20'__colorallblocks__'%3Bstyle.innerHTML%20%3D%20%60*%20%7B%20background-color%3A%20rgba(255%2C0%2C0%2C.2)%3B%20%7D*%20*%20%7B%20background-color%3A%20rgba(0%2C255%2C0%2C.2)%3B%20%7D*%20*%20*%20%7B%20background-color%3A%20rgba(0%2C0%2C255%2C.2)%3B%20%7D*%20*%20*%20*%20%7B%20background-color%3A%20rgba(255%2C0%2C255%2C.2)%3B%20%7D*%20*%20*%20*%20*%20%7B%20background-color%3A%20rgba(0%2C255%2C255%2C.2)%3B%20%7D*%20*%20*%20*%20*%20*%20%7B%20background-color%3A%20rgba(255%2C255%2C0%2C.2)%3B%20%7D*%20*%20*%20*%20*%20*%20*%20%7B%20background-color%3A%20rgba(255%2C0%2C0%2C.2)%3B%20%7D*%20*%20*%20*%20*%20*%20*%20*%20%7B%20background-color%3A%20rgba(0%2C255%2C0%2C.2)%3B%20%7D*%20*%20*%20*%20*%20*%20*%20*%20*%20%7B%20background-color%3A%20rgba(0%2C0%2C255%2C.2)%3B%20%7D%60%3Bdocument.body.appendChild(style)%3B%7D%7D)()

To color all blocks in a page.

But if the equivalent exist as a Firefox add on, I use that instead, because it bypasses many restrictions.

In this article, the video BM can be replaced with the excellent "Enhancer for Youtube", at least on Youtube: https://addons.mozilla.org/fr/firefox/addon/enhancer-for-you.... Just being able to change the speed of the video using the mouse wheel is a god send.

Same for archive.li or the way back machine.

But I'll steal the space scroll one, this red bar is nifty.