How do I put my document in strict (standards-compliant) mode?
In short, you add a DOCTYPE. An XHTML or HTML Strict DOCTYPE will accomplish it just fine in all browsers. If you want to see the details for each browser, see below:
IE 6 (Windows): MSDN on !DOCTYPE Switching
Mozilla and Safari: Mozilla's DOCTYPE sniffing
Opera: DOCTYPE Switching in Opera
IE 5 (Mac): Eric Meyer's Summary