2.1. XUL Means Boxes
When learning HTML, <P> tags and heading tags like <H1> are a common starting point. Only after some experience do you realize how invisibly powerful the <SPAN> tag is. An <H1> tag, for example, is just a <SPAN> tag with some CSS styles applied, like display:block and font-size:large. It's not the only such tag either—many are just <SPAN> plus some styling. HTML does not teach <SPAN> first because a plain <SPAN> tag doesn't appear to “do” anything; it's normally invisible.
XUL's <box> tag has the same role as HTML's <SPAN> tag, except that <box> is necessary right from the beginning. It's important that you master your XUL boxes straight away. Listing 2.2 is an XUL fragment showing typical use of boxes to structure content. ...
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