W3C validation is not everything. This is not the end of the world if your site does not check. The W3C validator was created at a time when only a few of the few cared about standards. It was a way to show the world that your site adhered to standards. In the end, your users probably don't care. They are there for content. These are other developers who do, mainly. It is also a quick check to see if there are any blatant errors, such as tags that are not closed.
If you buy a wooden frame, do you care what kind of coniferous wood they used for the frame? You probably care more about which color paint will be used. However, good practice will use spruce for bearing areas and something softer as spruce for normal areas. As long as it passes the test (does not crash), and this is a good build quality, you are probably happy. W3C validation is a bit like this (although I know the analogy is a bit wrong ...)
Just keep in mind that the -webkit- and -moz qualifiers are browser specific and more or less testable.
If this is sometimes important for your layout, then do not use them.
For example, I often use -moz-border-radius and -webkit-border-radius , but always include the usual border-radius . The lack of rounded borders in IE does not matter to me, because usually it does not violate anything. It's just not that pretty.
This follows the principles of graceful degradation and is good practice with CSS.
In the end, you have to do what is right for your audience. Web development is always about accepting and accepting ... compromise. I know that we all want to write everything perfectly ... but with the fragmented nature of the network, this is not real.
The good news is that it is much bigger than before.