To be honest, I didn't previously bother with bookmarklets because I thought they were cool, but lacking in practicality. Until I saw this...
I've replicated the 'edit styles' bookmarklet here. Just click on it, or drag it to your bookmark toolbar if you think it's useful (don't think too much - it IS).
» edit styles bookmarklet
With this, you can experiment with the CSS styles of your website(s) for those 'what if?' moments. You can also change how a site looks to your liking (too small font sizes, glaring background colors) at the click of a bookmarklet and some CSS editing.