Section 11 Text of the spec states:

"5.If the maxWidth argument was specified and the hypothetical width of the inline box in the hypothetical line box is greater than maxWidth CSS pixels, then change font to have a more condensed font (if one is available or if a reasonably readable one can be synthesized by applying a horizontal scale factor to the font) or a smaller font, and return to the previous step."

Currently the way the spec is defined, readability of text is difficult in the above situation. Further, the text reasonably readable is not very precise. Consider the following code:

ctx.font = '1000px Arial';
ctx.fillText("HTML5 Canvas 2D Context", 0, 300, 450);

IE and Firefox follow the spec, and the text is quite unreadable. Chrome and Safari do not implement maxWidth. Opera, however, renders the text at the font size that will fit in the maxWidth without scaling.

Operas implementation here seems to make most sense  as it is readable. This text should be updated like so:

"5.If the maxWidth argument was specified and the hypothetical width of the inline box in the hypothetical line box is greater than maxWidth CSS pixels, then change font size to a size that will fit in the maxWidth width without scaling the font, and return to the previous step."