I just developed a site in html5 and css3, but IE9 is not translating the styles at all. Looking around on the internet i see sites claiming that IE9 supports html5 and css3 for 99% - 100%.
Is this true or not?
First, note that CSS3 is not finalized.
Second, as a more direct answer to your question: IE9 supports some of the features from CSS3, but not all. There's a compatability list on quirksmode that provides more info. Also, resources like MDN keep track of browser compatability, see for example the text-shadow compatability list.