The text is rendered by the browser and some browsers don't scale text smoothly. I think what happens depends on which browser you're using. IIRC I filed an issue with at least Safari for text scaling not being smooth, but nothing much has changed.
That's ok. I'll try spritefont. But since you're here, could I ask you a quick insight on another thing? When I change the window size the drawing canvas gets slightly distorted, like 1 pixel smaller and create that tearing effect between images.
Is it possible to avoid that? I tried all Display settings combinations but nothing helps.