There are a couple of things you need to bear in mind when editing popup topics because of the different ways they are handled in different formats.
The plain-text popups used in HTML Help are stored in an internal text file in the HTML Help CHM file. This is the file you need to reference in your application calls to field-level popups. By default this file is called CSHelp.txt, but you can change this file name in Configuration > Publishing Options > HTML Help > Popup Topics.
In HTML Help the width of popup windows is controlled by the help viewer on the basis of the amount of text and the user's screen width. Since this system was designed a long time ago it does not allow for modern computers with wide-format screens and multiple monitors. When normal popups are displayed on these computers the popups can be much too wide, which looks terrible.
There are a couple of simple tricks for solving this problem, which are described in Creating popup topics in the Basic Procedures section.