The website is an explanation of the law, not the actual law itself. I'm sure the actual law does not require sales tax to be included in the displayed price.

Since he specifically mentions grocery stores, groceries aren't taxed.

TIL that's a thing. Illinois has a lower tax rate on groceries but still taxes them.

Also, grocery stores sell a lot more than just groceries.

State sales tax laws are all over the place; in many states the general idea is "don't tax necessities" which leads to silly things like buying a sandwich is tax free (like socks in Minnesota), but if you ask them to warm up the sandwich BEFORE sale it is taxed. If you ask them to warm up YOUR sandwich after sale, it's tax-free.

Cool, TIL grocery taxes exist.

Or it could be a state like Oregon where there is no sales tax on anything.