The following jsfiddle works correctly in chrome but not in firefox(at least in 16 and 21) (Look for size of rect) http://jsfiddle.net/ej5Zk/9/
<svg>
<g>
<rect y="0" x="0" width="400" height="463" />
</g>
</svg>
Actually it works incorrectly in Chrome and correctly in Firefox.
You generally want to set a width and height when embedding SVG in html
Per the SVG Specification
The ‘width’ attribute on the outermost svg element establishes the viewport's width, unless the following conditions are met:
Under these conditions, the positioning properties establish the viewport's width.
Since all the above conditions are true you need to use a style which means an explicit width since that maps to style, or a CSS width property. It seems most other UAs ignore the above specification requirements.