Companies try to distinguish themselves from one another by introducing new tags and browsers that interpret those tags. It's great for the user but rough on standardization. If that vendor ultimately establishes the de facto standard, then all is well. If not, content will eventually be lost and rendered unreadable. The only way to ensure uniform adoption of standards is to apply conformance tests to the use of the standard and browsers.
Conformance tests are where the rubber meets the road in the world of standards.
It is only by passing a conformance test that a document or browser can truly claim to comply with a standard. This type of branding is well known in the commercial world also, for proprietary specifications. Multimedia databases, books for the blind, talking books, printed text, and so on can be created from a single content source if that content is wellstructured. Remember that using your content in many ways makes your project more profitable and efficient. Of course, that does not mean that SGML is the only way to structure documents.