A web page doesn't have to look exactly the same, or be experienced exactly the same in all browsers. When designing and building websites, it's to incorporate the advantages of modern web techniques for those browsers that support them, as long as the design and build is done in such a way that the core content and functionality is available to older browsers too. u
Web standards, best practices and code conventions are the three different levels of coding goodness required to build a good quality site with respect to its client side code.
Web standards describe the flavour of code to be used to build the website. These are the set of finalised or emerging standards from bodies that are recognised as owning the standards for a particular web technology. These are the W3C, WHAT-WG and ECMA.