CSS web page layout misalignment: CSS width calculation

Why Calculate Width
Calculating the pixel width of web pages is for the sake of CSSweb page layout neat and compatible. It is common for us to calculate the width of the entire page when we lay out the left and right structural web pages or use padding and margin layout. If we do not calculate the width, whether the width is too large or too small, misalignment problems will occur.

How to calculate CSS width
Example 1: We calculate the layout style of a left and right structure.
If the total width is 400px, then the sum of the left and right should be less than 400px, then we may have 300px on the left and 100px on the right
Correct code:

The above is the correct total width of the left and right structures, which is exactly equal to 400px

Error:
If we keep the total width unchanged, the left side is 300px, and the right side is 120px, then the total width exceeds 20px. Let’s see what problems will occur. The DIV+CSS code is as follows:

We can see from the picture above that because the total width is about 20px, the left and right structures cannot be flush, and the right side falls down.
In this way, misalignment compatibility issues arise. Generally, in practice, due to our negligence in calculation, when the difference is small, it is usually 1px-2px, which will not be discovered by us. Therefore, to eliminate misalignment compatibility, we can start with width calculation.

Example 2: There is an example of 1px border in the left and right structure
Generally, there is a 1px border in the left and right structure, and then some borders are added. When we set the left and right structures, we need to calculate the width of the border and the width of the left and right structures together.
Correct example:
CSS and html code are as follows: