Exactly. I recently made a few webpages specifically for one job offer to go with my cover letter/resume. Unfortunately, these pages have never been opened…
I couldn't think of the best way to do it: a very short cover letter basically saying "open that webpage to see my real cover letter" might just annoy the recipient; a somewhat regular cover letter (my choice then) with an invitation to open a link still needs to be distinctive enough to convince the recipient to click the link.