But when someone is using a tablet (with no local dropbox folder) then the external hyperlinks does not work.
The above part of your question is little bit more concrete. You still should clarify what exactly you mean "external hyperlink", it's still unclear. Could be any link to other site or just a link to other file in your Dropbox account. Links to external sites should work always. If there something doesn't work, it's probably some fault of you. I'm going to assume you are trying to point to other file within your Dropbox account. As you already mentioned, there is no local folder and even more the mobile Dropbox application doesn't provide a way to directly access account resources. That's way other account resources left inaccessible, till you explicitly select them using the application. There is no other supported way currently, unfortunately.
Actually, I didn't say exactly this! I have said that they are not directly accessible, not that they can't be accessible at all. 😉 The key moment here is fact that shared links are redirect links, i.e. not something pointing directly to the actual resources. The actual links are temporary and are changing on regular basis. That's matter of security. The redirect link, when referenced, redirects correctly to the actual link. When you take this in mind, everything is possible. 🙂
Just make sure everything needed for your page to display correctly has shared link (i.e. all resources - HTML, CSS, images, etc). Everywhere in every one HTML use absolute link path, not shorthand! For example if you use file structure like:
Here links (and not only) are illustrative only. Real links should point to the real resources (i.e. to "css/mystyle.css" and "images/avatar.jpg" in this example). 👆 Once again you can NOT refer to any resource residing in Dropbox directly; you should use shared links instead. Direct referrings will work only on places where the particular resources are in sync local (your local computer/laptop for example, but also local on some web server when configured properly).