If you'd like to show some HTML content on a Shopify product page ONLY if the product has a specific tag, the following is the trick that will help you (hopefully!).
My use case: In one of my Shopify stores, I wanted to automatically show a different set of Frequently Asked Questions and Testimonials for each type of product I had. To accomplish this, I decided to tag each piece of content with the type of product it was. Then I wrote up some simple HTML to display for each product type. Then, I inserted the following for/if statement into the product-template.liquid: