The universal language in internet is HTML, therefore you have to make sure all your web pages are written in the most easily understood HTML codes (a.k.a the most commonly used HTML document type).
Using a new technology that redefine standards is cool, but don't forget there are still a lot of older version browsers out there (don't be surprise, try check your web stats and you will notice that). You should make sure your web pages are able to display properly on all older and newer browsers, I am sure you don't want to miss out any of your potential customers by showing them a corrupted web page on their browser.
Other than the old/new browser issue, be sure your HTML codes are compatible across different web browsers. Although every web browser understand generic HTML codes, there are slight difference in certain browsers when interpreting a same set of HTML codes, you must test thoroughly and make sure your web pages are showing exactly as they should across all the commonly used web browsers to be safe.
One extra note, if you really need to use javascript, place the javascript codes in an external file and let the HTML web pages contain just HTML codes. By the way, search engine crawlers ignore javascript codes when analyzing web pages, so using javascript in your web pages will not help search engines understand them any better.
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