Fully Valid HTML & CSS

From the same site discussed in an earlier post about CSS the navigation the Tongkat Fitness website has completely valid CSS and HTML! this seems quite good considering the creators of the  Showroom Shopify template said they used valid html and CSS where ever possible and still had some none valid CSS in there..?? can anyone tell me a fix for this?

CSS Navigation in Shopify

Can anyone tell  me how the image replacement CSS was applied to this site it has the look of the Shopify  Showroom template with the ajax cart and that but i cant work out how the designer gave the navigation id's and had it use some type of page id to give the [current page] highlight to the navigation.