When reading this question, i was wondering why anybody uses ex as a measurement for the width of an element. I understand that ex represents the height of a lowercase x, so i can use it to define the height of a input-box - for example.
But why would anybody use ex for the width of this box? Isn't that possibly wrong depending on the type? And why shouldn't I just use em in this case?
The height and width of elements are usually best specified considering both at the same time, to achieve "pleasing" ratios (e.g. 16:9, or the golden ratio). So just because ex is specified in terms of a height, it doesn't make it less useful for specifying a particular width.
And you shouldn't use em instead, since it's the width of the upper case M. It's highly unlikely that the two measurements are the same.
As a corollary, you'll find plenty of example layouts that specify heights in terms on ems, which is again because they're considering the ratios between heights and widths.
Since ex refers to height, rather than width, using it to set a width seems to be simply an error, imho. You should just use em to make the width relative to the text's width.
Hope this helps. :)
What @Andrei Bârsan said and, further, because few characters are as wide as 'em', you can use 'en' as a width specifier. 'en' will give you the width of any digit.
