The Microsoft HTML Help viewer, which is based on Internet Explorer, supports CSS (cascading style sheets); however, not every CSS feature is supported. Although CSS is probably the best way to set display properties for HTML Help, you might have to experiment. If a CSS feature you try does not seem to work, check the HTML Help Workshop on-line Help, or check with Microsoft to make sure HTML Help supports that feature.
For on-demand font resizing via the Font button (see §18.4.2 Adding tabs and toolbar buttons to HTML Help), CSS font sizes must be in relative units: em, ex, or %. For best practice, use em. By default, DITA2Go generates CSS entries using absolute pt units for font size and line height. To change the units, see §31.8.2 Specifying CSS size values and units of measurement.
§31.4.1 Specifying CSS options in a DITA2Go configuration file