CSS Gotchas - Part 1

TLDR: 2 CSS Gotchas today. First, the default CSS box model will cause an overflow if a box contains 2 boxes that are each set to 50% of the outer box’s height or width if either of the boxes contain a margin, border, or padding. Second, if you are not aware of vertical margin collapsing, you see unexpected results when the vertical margins collapse between 2 vertically stacked boxes.

50 percent plus 50 percent is greater than 100 percent

Here we have a simple box with 2 boxes inside of it. Each of the boxes are 50% high, which using normal math would be 100% but we are using CSS math.


It looks kinda ugly because it overflows and displays some scroll bars.  As it happens, the CSS box model puts the padding, border, and margin outside of the height and width measurements. This means that height height of the inner box is really 50% of the outer box plus 1 pixel border on the top and bottom and a 2 pixel padding on the top and bottom. For many applications, this can be easily fixed by adding the CSS tag “box-sizing: border-box;” which will tell the browser to include the padding and border inside of the 50% instead of adding it to the 50%. Here is what that result looks like.


The margin is still always outside of the box. If you want to also have a margin and handle it properly, you will have to use a CSS formula for your height and width. This looks like “height: calc(50% - 2px - 4px - 4px);” and “width: calc(100% -2px -4px - 4px);”


If you look at this result you will notice it still doesn’t look right. That brings us to the bonus gotcha. Vertical Margin Collapsing. The margin between the 2 boxes is set to 2 pixels for each box. Vertical Margin Collapsing turns that into 2 pixels total, instead of 2 pixels each by taking the maximum of the 2 margins (2px since they are both the same).

The quick fix is to adjust the formula to make the inner boxes a little taller to compensate. Since the 2 margins of 2 pixels each collapsed into 1 margin of 2 pixels each, we need to make each of the inner boxes 1 pixel bigger.


Today we looked at a couple of possibly unexpected gotchas in the CSS box model. To get the expected results, we switched the box model using “box-sizing: border-box;”, or we used a “calc” command such as “height: calc(50% -4px);”

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David Walker

David Walker is a Secure Software Consultant, a Certified Secure Software Lifecycle Professional (CSSLP), and a Professional Scrum Master. He believes in secure and reliable software and productive happy teams. He lives in Orlando with his lovely wife Lynn and his 2 dogs.