Are you typing this content while you're in the code mode of the editor? If this is the case you should first switch back to the on-code view (the rendered result) before showing. It'll save all the data correctly then. If you do not switch from the code to the rendered output the data goes lost.

Thank you for your advice; but I don;t know what are the "on-code view" and "code mode of the editor"? I type this content after I cilck "edit" and I can see the content I type in on this field, but when I click "save" , the content in field "Html" disappear; in addition, I use it on Odoo8.0 before, all is ok; and now I use it on Odoo9.0.

Hi vx2008
I'am currently facing the same issue. Have you resolved it. If yes, could you please explain, how.