You can apply rich text formatting by manually adding the following HTML-like tags to fields in your Zotero library:

• <i> and </i> for italics
• <b> and </b> for bold
• _and for _subscript
• ^and for ^superscript
• <span style="font-variant:small-caps;"> and </span> for smallcaps
• <span class="nocase"> and </span> to suppress capitalization rules (e.g., for foreign phrases within English titles)

Zotero will automatically replace these tags by the specified formatting in bibliographic output. E.g. “<i>Pseudomonas aureofaciens</i> nov. spec. and its pigments” will become “Pseudomonas aureofaciens nov. spec. and its pigments”.

Note that if rich text formatting has to be applied indiscriminately to entire fields (e.g. a style guide may dictate that titles should be in italics), you can modify the relevant Citation Style Language (CSL) style (see the CSL documentation and the CSL field formatting options).

A future version of Zotero will allow visual rich-text editing without manually adding HTML tags.