The following HTML elements are supported:
Bold: <b>, <em>
Italic: <i>, <cite>, <dfn>
25% larger text: <big>
20% smaller text: <small>
Setting font properties: <font face=”font_family“ color=”hex_color”>. Examples of possible font families include monospace, serif, and sans_serif.
Setting a monospace font family: <tt>
Strikethrough: <s>, <strike>, <del>
Underline: <u>
Superscript: <sup>
Subscript: <sub>
Bullet points: <ul>, <li>
Line breaks: <br>
Division: <div>
CSS style: <span style=”color|background_color|text-decoration”>
Paragraphs: <p dir=”rtl | ltr” style=”…”>
Taken from https://developer.android.com/guide/topics/resources/string-resource#StylingWithHTML