background-color: rgba(255, 0, 0, 0.5); as mentioned above is the best answer simply put. To say use CSSĀ 3, even in 2013, is not simple because the level of support from various browsers changes with every iteration.
While background-color
is supported by all major browsers (not new to CSSĀ 3) [1] the alpha transparency can be tricky, especially with Internet Explorer prior to version 9 and with border color on Safari prior to version 5.1. [2]
Using something like Compass or SASS can really help production and cross platform compatibility.
[1] W3Schools: CSS background-color Property
[2] Norman's Blog: Browser Support Checklist CSS3 (October 2012)