You'll need to check that your compose messages in Rich Text / HTML setting is enabled.
Log into webmail and go into "Settings." Under the "Compose" section make sure the box next to "Use Rich Text / HTML Editing" is checked. Be sure to "Save" the enabled setting. This should resolve the issue.
If you are a Safari users, this setting is not currently available but will be soon. In the meantime, you can enable the setting by logging into webmail via a different browser (e.g., Firefox) and following the above steps. After you've done that, the next time you log in via Safari, the issue should be resolved.