I care about your privacy. This webpage uses your internet browser to perform the extraction, so your document never leaves your computer.
In documents created with Zotero, references must be stored as "Fields", not "Bookmarks". This can be changed for existing documents through the Zotero document preferences. References from documents last used with Zotero 3.x or 4.x can only be extracted if the "Store references in document" option was checked (this option no longer exists in Zotero 5.x, as Zotero now embeds item metadata by default).
For maximum accuracy, use the CSL JSON format, which represents a carbon copy of what's stored in your document. All other formats involve a format conversion. CSL JSON is not widely supported, but can e.g. be imported into Zotero. Just click the "Download" button and import the downloaded JSON file via "Import..." in Zotero's File menu, or click the "Copy to Clipboard" button and use Zotero's "Import from Clipboard".
Did you find this free tool useful? Consider supporting my open-source work on Patreon for as little as $1/month, or buy me a coffee via PayPal.
For feedback, or to report any issues, please file an issue on GitHub or ping me on Twitter.
This open-source tool uses a number of open-source libraries, including Lars Willighagen's awesome Citation.js library for reference format conversion.