A lot of people fail to check their code, and although it rarely essential, it's considered good practice. Here is a quick and easy way you can do this from within Flux.
This is a stand alone version of W3C's HTML/XHTML markup validator.
First download a copy from here http://habilis.net/validator-sac/ and place it in your Applications folder.
Then in Flux go Flux -> Preferences -> Browsers -> Find Browsers to add it to the list.
Now from within Flux you can use Page -> Preview -> Validator-SAC anytime you wish to check your code.
It's free, quick and easy to use. Why not install it?