Safari 4 breaks when HTML5 section, nav, or article tags are used.

I built a website in HTML5 and it runs beautifully in everything but Safari 4 (and I'm assuming old versions of Chrome). When I change the <header>, <nav> and <article> tags simply to divs, I see the functionality coming back with each change. Apart from going back to XHTML (which I prefer to not do since I'm deploying my site as a child theme in WP on a HTML5 framework), is there any way to get around this?

Besides, I thought Safari 4 was a pioneer of basic HTML5 support. Header, nav, and article tags doesn't seem to be asking for a lot - not like I'm asking it to render canvas features. Any help would be much appreciated. My client isn't willing to update her browser.

Try setting display:block for all three elements at the top of your CSS. Webkit and Gecko browsers, even old ones can use basically anything as a tag if you define a display property for them.