HTML5 is the current standard so will give you the best footing going forward ('future-proofing' as far as you can). Old browsers will recognise the longer established tags fine - like paragraphs, tables, divs, headings, spans, anchors, etc. Only the newer HTML5 tags will be unrecognised - e.g. article, section, header, footer, nav - but you can serve a JavaScript shim in those cases, which will make IE7/8 play nicely with them too: https://www.google.co.uk/search?q=html5shiv - put it in the <head> section of your markup, between IE8 conditional comments so you're serving it only to the browsers that need it;
Code:
<!--[if lte IE 8]><script src="path/to/html5shiv.js"/></script><![endif]-->
If you're not using new HTML5 elements, you don't need the shim but you can still use the HTML5 doctype to benefit from modern standards/consistencies.
For reference, if you're ever unsure about which HTML5 element or CSS property is supported in various browsers, head for the 'Can I Use' website http://caniuse.com/ type it in the search box and out will pop all the compatibility stats telling you where it's supported (or not). It's really useful
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