An empty rectangle is likely to indicate that your output contains a character which your display apparatus (*) can't display properly. Alternatively, it could be an empty rectangle character. There's no chance anyone can help you solve this without seeing your input and your XSLT code; it's also much more productive to say what the actual character in the HTML file is, rather than saying what it looks like on some unspecified display device.
Some software on some operating systems displays line endings strangely, for example some older Windows software won't display files that use Unix line endings.
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference