Reason
Why?
A deprecated tag or attribute is one that has been outdated by newer constructs
.
A number of tags and attributes have been deprecated in favor of CSS: the font and center tags and the bgcolor attribute, for instance.
Proprietary elements have also been disallowed. They were developed by browser vendors, each for its own browser. A proprietary element is, in a sense, the "property" of a vendor; hence, proprietary. It cannot work well if at all across other browsers; hence, invalid.
