In HTML5 publications you can create frames which will contain standard HTML elements. This allows you to embed items such as paypal buttons, Google Maps. Twitter, Flickr or even RSS feeds easily.
Simply select the HTML Object from the Browser flyout on the Tools toolbar and then draw the object as you would any other Opus object.
Opus will set the size and position of the DIV containing your HTML but you must enrsure that the content fits and that the HTML is error-free.
You can then enter your HTML into the edit box provided and Opus will automatically include it in your published file.
Note:
Opus does not check the HTML in any way so you must ensure that any script entered here works as required. If your HTML5 publication fails to run or you get a black screen then this may be because the embedded HTML has errors.