[css21] background-color of canvas

How is the color of the canvas determined when the colors of the html and body elements are not specified? In section 14.2, "The background", the spec says that the background of the root element becomes the background of the canvas and
covers the entire canvas. It also says that for documents whose root element is an HTML "HTML" element  that has computed values of 'transparent' for 'background-color' and 'none'
for 'background-image',
user agents must instead use the computed value of the background properties
from that element's first HTML "BODY" element when painting backgrounds for the canvas. However, if the background properties (color and image) are left unspecified for both the html and body elements, they will receive their initial values of 'transparent' and 'none'. So how does this translate into a white background for the canvas? Shouldn't the spec say something like, "If the background properties of html and body are unspecified, the canvas background should be painted white." Or is this just one of those unspecified things that's left up to the browser implementers?
Thank you.