calc() allows to calculate sizes

Read more about css3 calc().

Testsuite

This element's width should be: 100% / 2 - 50px

Support

Yes

Those browsers support calc():

Firefox 4 beta4

No

Those browsers doesn't support calc():

Internet Explorer 6 - NO
Internet Explorer 7 - NO
Internet Explorer 8 - NO
Firefox 3.6 - NO
Opera 10.5 - NO
Opera 10.6 - NO
Chrome 5 - NO

Unknown

Those browsers are not tested or not released yet:

Internet Explorer 9 - UNKNOWN