HTML and CSS Reference
Finally, once all tests are complete and all required polyfills are
loaded, the app.js file is loaded and your application can start
up properly, safe in the knowledge that even if it's running in
a dirty old browser like IE6, it can still use the latest, snazziest
technology like sessionStorage and JSON.
We've seen that it's possible, and relatively simple, to make
HTML5 apps work in old browsers by performing feature detec-
tion (as by design, most—but sadly not all—HTML5 features can
be programmatically detected) and patching in support with
helper script. However, it's important to be aware that polyfills
probably won't perform as well as their native counterparts,
and that older browsers like IE6 will be further slowed down by
these cases to avoid polyfilling and provide a completely differ-
ent fallback solution.