Saving a modified epub in Sigil correctly

Using Sigil 0.9.9 on my Windows box, I have edited some text in an epub. I have put span tags around some words and put CSS into its css file for grey background.

Sigil shows those words with grey background.

Now, when I save this file with Sigil and open it using Firefox's EPUB Reader, that part is not shown with grey background.

Did I miss anything to achieve the effect of saving my modifications?