If I set a font-size on the body element, will that size also be 1em?
In other words, if I do:
body {
font-family: Arial;
font-size: 16px;
}
Can I be sure everywhere I use 1em it will equal 16px?
If the above is not the correct way, how else should I set a base font?
Sorry if base font is not the correct term. I’m not sure what this technique is referred to as. I believe it is similar a reset stylesheet, which overrides the browser's default styles.
Answer
What you are looking to achieve can be done with the rem unit.
An em unit is “Equal to the computed value of the ‘font-size’ property of the element on which it is used.” In your case 1em would be 16px on the body element. But, if you change the font-size on any element, 1em on that element will equal the font-size of that element. If you use ems on font-size, it is relative to the font-size of the parent element. Thus if p was the child of div, and font-size on div was 12px, then font-size: 1em on p would be 12px.
On the other hand, a 1rem is always equal to the computed value of the font-size of the root element. In HTML this is the html element. Thus the size is always consistent. See http://www.w3.org/TR/css3-values/#rem-unit