They restrict it to Safari because the dire truth is that so far, Safari is that one browser that does the STANDARDIZED HTML5 portions best. Chrome hits the second place not far behind, not surprisingly since it also uses the WebKit engine. Firefox however, by many peoples' experience - and, again, using HTML5 demos that do not use vendor-specific CSS magic, APIs and whatnot - simply fails at running most of these demos.
add.: to the downvoters... really, just load the page up in Chrome, check the demos, then load it up in Opera (with User Agent spoofing) and check the demos there, THEN try loading it up in Firefox, and you'll see for yourself whether it's a case of these HTML5 demos being "proprietary", "Safari only" or "broken", or if it's a case of Firefox just not cutting it yet. (Spoiler: it's a case of Firefox just not cutting it yet; almost all of the demos work identically in Chrome and Opera.)
add.: to the downvoters... really, just load the page up in Chrome, check the demos, then load it up in Opera (with User Agent spoofing) and check the demos there, THEN try loading it up in Firefox, and you'll see for yourself whether it's a case of these HTML5 demos being "proprietary", "Safari only" or "broken", or if it's a case of Firefox just not cutting it yet. (Spoiler: it's a case of Firefox just not cutting it yet; almost all of the demos work identically in Chrome and Opera.)