Today is a holiday in the US, so we're republishing this topic from a newsgroup post not too long ago.
Summary:
You can add the User.visEquivTitle cell to your shape and provide a string
that will display as a tooltip in HTML web output. The shape must also have
at least one custom property for the tooltip to be visible.
Steps:
1) Select the shape in your drawing that you want to add a web tooltip for
2) Go to Window > Show Shapesheet
3) Go to Insert > Section, check User-defined cells and click OK
4) In the newly created section, click on the row name User.Row_1 to select
the title of the row
5) Type visEquivTitle and press Enter - you should now see
User.visEquivTitle for the row
6) In the Value cell of that row, enter the text you want to display
surrounded by quotes (e.g. "Hello World")
7) If you instead want to display the same tooltip as the Shape ScreenTip
enter =Comment (no quotes)
8) Go to Insert > Section, check Custom properties and click OK
9) You can fill in the custom property Label and Value if you want this
information to be visible
10) If you don't want the custom property to be visible, enter TRUE in the
Invisible cell
11) Close the Shapesheet window and repeat for any other shape you like
12) Choose File > Save as Web Page to create the HTML output
As you can see this is a bit involved. The capability was designed more as
a developer feature than an end user feature. We recommend tying the Shape
ScreenTip to the User cell so that you see the same tooltip in both Visio
and the web output. It should also be possible to use Macro Recorder to
automate these steps. If your shape has a custom property but does not have
the User.visEquivTitle cell, Visio will display the shape's text in a
tooltip instead.