To add a website to your collection, just click the saved bookmarklet while visiting that site.
Unfortunately not all sites can be saved in this way, e.g. sites from github.com. This is because of current browser's Content Security Policy defaults.
A simple app that hooks into the share intent for easier creation of bookmarks.
There exists an API. It is in beta stage and might change any time.
All browsers and bookmarking services should have the option to export your bookmarks in the Netscape HTML format.
You can import your existing collection of bookmarks here.
While we take great care to not lose any of your data, we recommend you to backup your collection from time to time.