Eric,
thank you for your reply. It obviously works in your example.
I tried it again, and I'm getting intermittent results - with the same browser, but in two different tabs.
I'm attaching two files (screen shots from the same post) which show different effects.
Here is the actual text as entered ([] substituted with <>):
<b>A single sneeze releases about 30,000 droplets, with <u>droplets traveling at up to 200 miles per hour.</u></b> Most droplets are small and travel great distances (easily across a room).
I'm using Opera browser on Windows 8.1, so maybe that's where is the problem.
EDIT:
One more thing. I tried it again (in the same browser tab), and the preview shows it correctly with underlining, whereas viewing the completed post shows it without the underline.