Safari > Develop > Disable Styles
Opera > View > Style > User Mode
It doesn't look like Chrome has an option by default, and I don't have IE on hand to test.
javascript:var%20i=0;if(document.styleSheets.length>0){cs=!document.styleSheets[0].disabled;for(i=0;i<document.styleSheets.length;i++)%20document.styleSheets[i].disabled=cs;};void(cs=true);