Precise meaning of 'en'?

How should an 'en' be interpreted?  Does it inherit its point 
size from current element, or the parent element, or HTML?

I.E. if one had

html {font-size: 8pt}
p {font-size: 12pt}
p {text-indent: 3en}
p.initial {font-size: 24pt}

Then how much would the first line of p.initial be indented? 

If instead the second line were p {margin-left: 3en} then what would 
be the size of p.initial's margin?

How does :first-line fit into this?

Michael Seaton(mseaton@informp.net)