<h1> to <h6> elements
Support for headings elements in HTML: <h1>, <h2>, <h3>, <h4>, <h5>, <h6>.
Outlook
Windows
2003
2007
2010
2013
2016
2019
Windows 10 Mail
2020-05
macOS
2011
2016
Outlook.com
2020-05
iOS
2020-05
Android
2020-05
Samsung Email
Android
6.0
Mozilla Thunderbird
macOS
68.7