When setting CSS font-size in pixels, does the value refer to the width or height of a letter?

Question

Does the pixel value of a font-size refer to the height of the font or the width of the font?

CSS:

.sixteen {
  font-size: 16px;
}

Answer 1

According to this article by Vincent De Oliveira, the font-size actually has nothing to do with the actual height of characters. It's not even the height of the inline element that contains text.

Of course, a larger value of the font-size property results in a larger text. But the pixel value does not directly translate into the actual size of text.

However, the pixel value of font-size sets the size of 1em to be used for setting margin, padding, etc.