I'm guessing it could deal with the css applied to the rest of the text in the sentence.
lets say this is your sentence css
font-family: arial, helvetica, sans-serif;
When adding styles in the markup it could null and void the specifications.
Also, if could be because an "a" element is added and you might need to see if it's conflicting with any universal rules to the "a" element.
I would give the <a> a class and just change the color of the type with css.
Given that you're using <font>, I'm assuming you aren't using any CSS. If you are, there may be another rule that's coming into play, so you'll need to show us your stylesheet. If you're changing the size of the text (and if you're not, why specify a size?), it could be that the font looks a different weight but is actually a different size.
That too haha.