Ok this is seemingly impossible to get right. I have a text box and a select box. I want them to be the same width exactly so they line up on the left margin and the right m
This is because the element has a slightly different default style to the
element e.g. the border, padding and margin values may be different.
You can observe these default styles through an element inspector such as Firebug for Firefox or the built-in one for Chrome. Futhermore, these default stylesheets differ from browser to browser.
You have two options:
I would go with option 1. because option 2. requires a lot of work and you'll end up with tons of unnecessary CSS. But some web developers prefer to start from scratch and have complete control.