Nah, you were right first time, sb.
<span> tags may appear useful, but they are meaningless.
They're certainly not 'the best way' to achieve what you're after, oto.
Use styled <em> tag as sb suggested and it'll emphasise the red word in other circumstances too.
Whenever possible, use meaningful tags in your markup, not meaningless ones.
Why are you emphasising the word in the first place?