Back to Diigo for a moment. It works with multiple browsers (I'm most familiar with the bookmarklet, the add-on for Firefox, the app for Android which integrates with Firefox etc., and the extension for Chrome). The web interface allows you to search based on various criteria, including texts that you have highlighted in pages that you have bookmarked. Super useful.
That seems reasonable.
Attention to wording: tags or labels?
So a post may be truly unread, or (maybe a future feature) read then marked as unread, and/or tagged/labelled as unread within the context of a post-specific user-specific bookmark.
Folks, consider changes that will be required to mobile interfaces.
Here, for example, the binary
[ ] I've read –
– which appears to be a heading or sub-heading to the obscurely-titled any menu.