Requirements

I modify HLI whenever I need it. On the other hand that means, I probably will not add things I don't need. But I'm open to feature requests. And it is one of my efforts to make HLI compatible with a wider range of server/browser environments.

Operating System
I'm working on WindowsXP® with XAMPP and on some custom Debian installations. Both work great.
Webserver
I never tried it with anything but Apache. Probably HLI will not work on other servers.
PHP Version
I'm working a lot on PHP 5.1.6 and PHP 5.2, so both should work. For some time now I have an eye on it to write code that doesn't require too up-to-date PHP versions and I would be glad to get some response on how it works on older versions.
However, PHP 4 will probably never work.
Browser
Tested with Firefox 1.5.10, 2.0 and Opera 9.10. IE 7 displayes the dumps but Javascript is not working yet.
Encoding
I do develop UTF-8 encoded applications whenever possible and HLI has not yet been well tested with other encodings.
Required PHP-Extensions
I try not to require too many PHP extensions but actually, I don't know exactly how much extensions HLI requires. Howevery, the DOM and XSL extensions are required – HLI generates pure XML that is transformed, only to HTML for now, but possibly you could have your dumps in LaTeX. A cutting edge GNU/Linux distribution seems to ship with a PHP compilation configured --without-dom. I have no idea why, but they also don't like Firefox 2.0