The HTML Editor now allows you to apply formatting based on the individual elements in your issue. While you can still highlight text and apply a format (i.e. Bold, Font Arial), a more precise way to do that now exists.
1. Place your cursor in the text (or element) you want to change.
This will then display the Element Tree at the bottom of the editor. Your text is nested inside these elements. In this example, my text is inside a font tag, that is inside a font tag, that is in a paragraph, that is in a cell, that is in a row, that is in a table body, that is in a table which is in the message body.
2. Click the element you want to change the format of
This will highlight all of the text in that element. Usually the element you want to click is the one to the far right and it doesn't have to be a font element (e.g. span, div, p).
3. Any formatting you change will be applied to that text
If your formatting doesn't take effect, it is because another element farther to the right on the tree is overriding it.