When on chrome and use `Save As`, in the dialog box that appears there is no `Web Page,Complete` format to select. I have tested it and it appears on Firefox. When changing from Kubuntu to Unity both Chrome and Firefox include this format.
Not an ark bug. Might be an issue in kdialog which is what chromium uses to show the file picker.
See https://bugs.chromium.org/p/chromium/issues/detail?id=431505 I checked kdialog, and it supports mimetype filters, e.g. kdialog --getsavefilename $HOME "text/html application/x-webarchive"