I have some links on a web page which, when the mouse is moved over them, cause a small box to appear with an explanation of the link. For some unknown reason, these small boxes are now transparent, even though they have a background colour set, and the underlying text shows through making the contents of the box unreadable. The page hasn't been changed in months, and this is a new phenomenon that occurs only in Internet Explorer.

How can I make the background of the small boxes opaque again?

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System 3 2000 Limited
www.system3-2000.co.uk

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System 3 2000 Limited
www.system3-2000.co.uk (but very out-of-date!)