Well, you can't show an html page in a dialog box, but to show an html page in a browser, do this:
LrHttp.openUrlInBrowser( url )
If you want to present a dialog box with a button or drop-down for selecting the URL.., then you'll need to create a view (see LrView namespace) assigned to the contents of an argument table, then call:
LrDialogs.presentModalDialog( argTable )
Thanks for your reply. Is there any third party library to get this done?
You can't wire 3rd party libraries into a plugin. I mean, a plugin can talk to an external application but plugins just can't display html in a dialog box no matter what they do... I mean, you could get clever and wire up a conversion to png then display as a picture, but that's the only way I can think of right now.