Browser viewer: Restore the default font as sans-serif

But it can now be easily overriden by either user-stylesheet
or book style sheet

src/pyj/read_book/footnotes.pyj

@@ -198,6 +198,7 @@ class PopupIframeBoss:
         self.frag = data.frag
         for name in self.blob_url_map:
             window.URL.revokeObjectURL(self.blob_url_map[name])
+        document.body.style.removeProperty('font-family')
         root_data, self.mathjax, self.blob_url_map = finalize_resources(self.book, data.name, data.resource_data)
         self.resource_urls = unserialize_html(root_data, self.content_loaded, self.show_only_footnote)
         update_settings(data.settings)

src/pyj/read_book/resources.pyj

@@ -2,9 +2,11 @@
 # License: GPL v3 Copyright: 2016, Kovid Goyal <kovid at kovidgoyal.net>
 from __python__ import hash_literals
 
-from dom import clear
+from elementmaker import E
 from encodings import base64decode, utf8_decode
 
+from dom import clear
+
 JSON_XHTML_MIMETYPE = 'application/calibre+xhtml+json'
 
 def decode_component(x):
@@ -228,6 +230,7 @@ def unserialize_html(serialized_data, proceed, postprocess_dom):
     apply_attributes(html, document.documentElement, ns_map)
     head, body = tree[1], tree[2]  # noqa: unused-local
     clear(document.head, document.body)
+    document.head.appendChild(E.style('html { font-family: sans-serif }'))
     resource_urls = {}
     load_required = set()
     proceeded = False