Thanks for your inquiry.
First of all, to be able to load HTML page via URL, please try using the same approach as suggested in the following post:
Moreover, upon processing HTML, some features of HTML might be lost. You can find a list of limitations upon HTML exporting/importing here:
Saving a Document
Secondly, Aspose.Words has its own rendering engine that is used to layout document into pages upon rendering to various “fixed-page” formats, like PDF, XPS, SWF, besides the same rendering engine is used upon updating fields. Please follow the link for more information:
Please let me know if I can be of any further assistance.