The HTML fields used in ServiceNow can use different editors depending on what is activated.
This article shows the different ways you can configure these HTML editors.
Learn about HTML Fields
There is a good wiki article about HTML fields in ServiceNow. Modifying HTML Fields
There are two different types of editors. htmlArea and TinyMCE. You really want to use the TinyMCE editor, that has a lot more options than htmlArea.
TinyMCE is open source editor, "the most advanced WYSWIYG HTML editor"" as their site indicates!
Switch to TinyMCE
In order to switch to TinyMCE, you adjust the Property, glide.ui.html.editor, and set to tinymce. Be warned that you should try this in a development instance first. Converting to TinyMCE from htmlArea may mean that you will need to redo images. If you have a lot of KB articles with images that use htmlArea, this can be a lot of work.
Add Buttons to TinyMCE
How to add addtional buttons to TinyMCE.
Type sys_properties.list in the left navigator bar
Search for glide.ui.html.editor.toolbar.line
Adjust line1 and line2 as desired. Here is my favorite setup (however it takes up more space in the editor)
Issues with HTML Fields
Why not just convert all the text areas in ServiceNow to HTML fields instead? There are some obstacles.
Activity Feed. Activity feed doesn't render HTML changes correctly
Email. Email may not display html information correctly without configuration.
More Space. More space is need to store HTML data. Although this is minor perhaps.
Form Size. Forms are bigger with HTML fields.