When you use your Order Printer Templates design with Shopify’s Order Printer app, you might see a message stating that "This template might contain invalid HTML" when hovering over the name of your template in the 'Templates' section of the app:
This message is placed by Shopify when code coming from outside their default templates is added to their app. It is not part of any existing issue with your Order Printer Templates design; it is simply a warning from Shopify stating that they are unsure if the template contains invalid HTML.
If you need further assistance or have any questions, our friendly support team is more than happy to help 24/7!