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Units of styling

So I am trying to write a little css to format a dropdown to match an input. The input has the following bubble settings:
horizontal padding: 10

and when hovered:
border width: 2
horizontal padding: 9

The reason the horizontal padding changes when hovered is because the border takes up width so you have to subtract that from the padding otherwise your text will shift slightly when hovered vs when not.

Given that the dropdown element doesn’t have the horizontal padding option, I am having to write some css, however when I use the following css:
<style> #padded {padding:0 10px!important;} #padded:hover {padding:0 9px!important;} </style>
It doesn’t match up with the input element, implying the units are different.

Any idea what unit I should be specifying in my css to get it to match?
Thanks in advance.