What does 1__qem mean?

Question

I reading through the default styling applied to HTML elements for Google Chrome, available here. I found this:

p {
    display: block;
    -webkit-margin-before: 1__qem;
    -webkit-margin-after: 1__qem;
    -webkit-margin-start: 0;
    -webkit-margin-end: 0;
}

What does 1__qem mean?

Answer 1

From the WebKit source: CSSPrimitiveValue.h

// This value (__qem) is used to handle quirky margins in reflow roots 
// (body, td, and th) like WinIE.  
// The basic idea is that a stylesheet can use the value __qem (for quirky em)
// instead of em.  
// When the quirky value is used, if you're in quirks mode, the margin will  
// collapse away inside a table cell.

More information on Quirks Mode: http://en.wikipedia.org/wiki/Quirks_mode

Modern sites should never be in Quirks Mode, so you're safe to assume that it's the same as em for all intents and purposes.

Answer 2

The sheet contains values such as "1__qem".

I don't really have a clue what these mean. The only reference I found about this is a post from Bill Brown on the CSS-Discuss list. It says:

I believe qem stands for "quirky em" and is a proprietary Webkit syntax used to refer to a margin which can be collapsed when the page is in quirks mode.

According to Tab Atkins, it is some "WebKit magic" :-)

Source: http://www.css-101.org/articles/base-styles-sheet-for-webkit-based-browsers/index.php