I would suggest that first of all all of your web pages include the decleration for HTML5, <!docytpe html>. Doing so will not cause any issues for your visitors.
From that point, its up to you on what HTML5 elements CSS3 styles to include. You can include some of them or none of them at this point. You have control.
When you want to include something, you simply need to research which browsers will support it. For example, say you include something, but it requires IE9 or later. If you need to support IE8, then you may need to provide an alternate solution for the browser(s) that do not support it. You may have to use modernizer.js or your own methods to detect the browser help you complete these tests.