Seems like Arma has very limited HTML features, no colors except for link, active link and bold text. Headings can have different fonts, see Epoch_GUI_ClientConfigs.hpp file for more info on this.

Depending on size of HTML, you might need to increase height of control to enable scrolling. You can do that in inside Epoch_main_config_html in before mentioned file. Height is calculated in percentages (42% = 0.42 * safezoneH)

TIP: Use filepatching to edit it live, file is reloaded when ESC is pressed