Nowadays, the majority of websites I see (and all of those I make myself) are written using HTML5, rather than the older HTML 4.01 standard.
All modern browsers understand the new semantic elements of HTML5 (the new structural elements, video, and audio tags) and even older versions of Internet Explorer (versions before Internet Explorer 9) can be served a tiny 'polyfill' to allow it to render these new elements.
Note
What is a polyfill?
The term polyfill was originated by Remy Sharp as an allusion to filling the cracks in older browsers with Polyfilla (known as Spackling Paste in the US). Therefore, a polyfill is a JavaScript 'shim' to effectively replicate newer features in older browsers. However, it's important to be aware that polyfills add extra flab to your code. Therefore, even if you could add 15 polyfill scripts to make Internet Explorer 6 render a site identically to every other browser, it doesn't mean you necessarily should.
If you...