Browsers will ignore any properties which they don't understand. If the property is yourown, how will the browsers know how to implement it? There is a convention for browser vendors to add propitiatory properties by prefixing them with a hyphen followed by a vendors specific identifier, follow by another hyphen, and finally the property (e.g. -moz-border-radius for round corners in Gecko browsers). It's done this way because there are no standard properties which start with a hyphen. Iirc, the currently recognized identifier include "moz", "o", and "khtml" for Mozilla, Opera, and Konqueror/Safari, respectively. MicroSoft, as usual, doesn't follow the convention, which may result in problems in the future.