I've been thinking about storing the logs in .txt-files (with the option to turn this feature on/off). This way, I could let the user download the logs directly via the browser.
I'm not familiar with how difficult it may be, so please tell me if I'm off base. But couldn't you offer the option for either text logs or HTML logs? I know that when given the option, I opt for HTML logs for the ease of reading.
While I haven't implemented the logging yet, I'm thinking about storing them in the JSON-format. Messages are already stored as JavaScript object, so I think this is my best option. And exporting them as .txt on demand, shouldn't be a problem either.