If you go to this page: https://www.caniemail.com/clients/

and select "check all", it'll show you the features that are supported by all the email clients, and separately, which features have mixed support.

These appear to be the few features supported by all clients:

- border-collapse

- font shorthand

- list-style-type

- cm unit

- em unit

- ex unit

- in unit

- mm unit

- pc unit

- % unit

- pt unit

- px unit

- vertical-align

- <del> element

- <div> element

- <h1> to <h6> elements

- <hr> element

- <img> element

- <p> element

- <pre> element

- <span> element

- <strong> element

- <table> element

- valign attribute

- JPG image format

- PNG image format