Hey @xponential 

Welcome to the community and thanks for asking about subject lines!

HTML isn't supported within subject lines, which makes formatting a challenge. A workaround is to use Unicode symbols; this site will translate your text into different formatting using various Unicode symbols.
 
However, I'd advise against using this, as there's not a way to guarantee that those symbols will appear consistently across devices. If a specific device isn't able to render a character, it will appear as some other symbol (often an empty square) instead, which isn't the best user experience.

Hope this helps!