A Brief History of HTML
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In addition to this official work on HTML, the browsers have been making their own additions to HTML. Some changes were eventually adopted into W3C HTML Recommendations; others remain proprietary coding aspects that only the individual browsers recognize. The browsers' versions of HTML changed, too, in a game of marketing and programming one-upmanship, hoping to lock Web developers into using one browser or the other exclusively.
Designing for the Web can be a confusing activity, indeed.
from Special Edition Using HTML 4: Appendix A
What's New in HTML 4.0
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