On January 19, 2011, the W3C unveiled the HTML5 logo. The moment that page opened, the image hit with a strangely familiar feeling: it looked less like a web standard badge and more like a poster from the kind of old studio films many of us grew up watching. It had that unmistakable rallying spirit.

You could almost read it as a proclamation to web developers everywhere: unite, endure the hardships, fear no sacrifice, and carry the HTML5 cause through to the end.
So yes, web developers should not only add an HTML5 badge like the one below to their HTML5 pages—covering things like connectivity and realtime features, CSS3 and styling, device access, graphics, 3D effects, multimedia, performance and integration, semantics, and offline storage—

more importantly, they ought to leave a declaration like this right inside the code itself:
<!--
.... This website is built in HTML5 ....
... ..... ......... ....... ......
.. MM MM NMMMMM MMM MMM..MM . ....
. MMMMMM...MM ..MMMMMMM..MM. ......
. MM77MM...MM . MM.M.MM..MM........
. MM MM MM MM. MM..MMMMM.....
. . . ....................
. . ================================....
. 7777777777777777777777777777777+....
. 7777777777777777IIIIIIIIIIII777.....
. 7777777777777777I77777I7I777777.....
. 7777777777777777I777I7I7I777777.....
. ..777777,,,,,,,,,,.........,7I777.....
. ..=77777,,,,,,,,,,.. .. .+II777...
...77777,,,,777777777777777II7777.....
. .77777,,,,777777IIIIIIIIIIII77? ..
...77777,,,,777777IIIIIIIIIIII77:.....
...77777:,,,,,,,,, .I7777.
...77777?,,,,,,,,,... ... .II777. ...
.. 777777777777777IIII= .II777. .
.. I77777777777777IIII....~II777.. .
...,77777,,,,77777IIII III77$ .
. ....77777,,,,77777IIII III777
. ....77777,,,,,,,,,... ... III77,. ...
. ....77777,,,,,,,,, .III77.
.. 7777777777,,,,...=7IIIII777.. ....
....$7777777777777IIIIIIIIII777 .
....I7777777777777IIIII7I777777.. ....
.... .7777777777777777777777$~... ....
... ...~$777777777$7. ....... ....
... . .........+....................
... ............ . . .. . . .. .
-->
A logo is one thing. Turning it into a banner, a badge, and even a comment in the source code—that is much more in the spirit of the moment.