Salesforce Support Scope for HTML Email Templates

Email Template Editors

Salesforce fully supports the standard functionality of the email template editors available in Salesforce, including formatting controls, merge fields, and related features.

HTML Templates Using Classic Letterhead

These templates are fully supported by Salesforce because the platform provides an easy-to-use HTML editor (where you can add images, links, and bullet points) and no HTML coding knowledge is required.
For HTML (using Classic Letterhead) templates, do not directly copy and paste the template body from a formatted document. Instead, write the body directly on the Salesforce template page using the formatting controls in the template editor.
If the text must come from a prepared document, copy it from the source (such as Microsoft Word) into a plain text editor (like Notepad), then copy from the plain text editor into the Salesforce template body. This removes hidden formatting codes from the source that may not display correctly in the recipient's email client.

Custom HTML Templates (Without Classic Letterhead)

Custom HTML templates are templates created from HTML code written outside of Salesforce and pasted into the HTML body field in Salesforce.
Custom HTML templates are not supported by Salesforce. Salesforce can assist with merge field functionality only. Any other issues related to template content — such as incorrect formatting, broken links, or invalid HTML — must be resolved by the customer and require web development knowledge.

Template Design Best Practices

Template design decisions should be owned by the organization's marketing team. Best practices include:

• Test the email template before using it in production.
• Preview the template in multiple email clients (Gmail, Outlook) and on different screen sizes (desktop, tablet, phone) before sending.

See Also:
Create Custom HTML Email Templates in Salesforce Classic 
Considerations for Using Content from External Sources in Email Templates