Tips for designing Shopify email notification templates
Use Components
See How to use Email Components.
When designing Shopify email notification templates, some of your email elements/rows are reused across different notification types. Hence using email components to reuse these elements/rows can greatly speed up and simplify the process of crafting email template.

For example, email header is used across all notification emails, so it's a good choice to save it as a component. Another example is email footer.

