[whatwg] HTML5: compatible with all legacy Web browsers

[Aryeh Gregor]

I think the meaning of "compatible with all existing browsers" here is
that HTML 5 does not *require* authors to break compatibility with any
existing browser.  Obviously some new features of HTML 5 will not work
in some existing browsers -- otherwise there could be no new features
in the spec!  But it's designed to support graceful degradation
wherever possible, so that authors can use many of the new features
without breaking compatibility with any existing browser.  This is in
contrast to its erstwhile competitor XHTML 2 -- XHTML 2 cannot be used
in any legacy browsers, ever.

Clearer wording might be like, "HTML5 pages can be written to be
compatible with all legacy Web browsers."  Of course, "all legacy Web
browsers" does need to be construed to exclude Netscape Navigator 3
and such.  If you really want to be picky, it could be "all legacy Web
browsers that still see significant use."