they work for all.
span tags are meaningless when taken away from a stylesheet. stylesheets are there to seperate style from content. take a look at your markup without the style sheet attached. if you use a styled <em> tag then your browser will still know to give emphasis to this word. if you're not wanting to put the word in red for emphasis then why is it highlighted in red? surely you are emphasising the word's existance on the page?
this can't be a theoretical question.