Twitter tells us in their release notes:
Better box model by default. Everything in Bootstrap gets box-sizing: border-box
, making for easier sizing options and an enhanced grid system.
This sounds so cool. But in real life this could lead to problems with old-fashioned CSS apps. They did not apply the new box-model to single elements, they changed it hard and global for everything:
*, *:before, *:after { box-sizing: border-box; } /* <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/box-sizing">box-sizing</a> */
When you use a hard-coded height or width in combination with a padding or border for elements, you need to check if it still looks the same.
Previously styled form elements are now a bit smaller if you don't fix them. When starting with new Bootstrap 3 stylesheets, there are no problems to be expected.