CSS3 relative units: em && rem

Em:

1. font-sizeof em relative to the parent element of the Element. For example, if a <div> 's font-size is "16px", then if its child element's font size is set to "0.75em", then its font size is calculated to be equivalent to "0.75 * 16px = 12px"

2.p { margin: 1em 0 }

Indicates the paragraph left 1em blank, then the specific P element font size if it is 12px, White is also 12px, if the font is 16px, white is also 16px.

Rem:

REM is relative to the HTML root element, which is the HTML element, such as:

html{
    font-size: 14px;
}
p{
    font-size: 2rem;  /* 此时p的字体大小为28px，即html的font-size和p的*/
}

CSS3 relative units: em && rem