Tested working with Firefox 9.x (with and without Stylish) and Chromium 16.something (without Stylish) on Ubuntu 10.10 Maverick. (Update: Re-tested with Firefox Nightly 28.0a1 and Chromium 30.0.1599.114 both using the latest Stylish extension on Ubuntu 12.04.) A happy Valentine's Day gift to the Simplenote users who have been http://help.simplenoteapp.com/customer/portal/questions/57081-mono-spaced-font for a while.

The Simplenote web app's borders in the screenshot have been removed by http://freestyler.ws/style/35008/simplenote-remove-border. Also seen in this screenshot: the Resurrect Pages, Pinboard.in, and Omnibar Firefox extensions, plus the DuckDuckGo search plugin. Update 2013-11-23: still works after Automattic acquisition and iOS 7-ish redesign.

If this style helps you out and you feel like externalizing your joy by giving away some money, please consider https://donate.mozilla.org/, the creators of the Firefox web browser, without which I couldn't have created this style. (Okay, http://getfirebug.com/contribute and helped, too.)