ninja33/odh (Online Dictionary Helper) is a Chrome/Firefox extension that lets you select a word or phrase on a web page or in a PDF document and shows the dictionary meaning. Dictionary entries come from two routes: online dictionaries and the built-in dictionary.
The core of this extension is connecting dictionary lookup with review study. When Anki and AnkiConnect are running, the (+) button in the definition window adds the selected word or phrase to an Anki note. Since not only single words but phrases can be selected, an expression whose meaning and context you checked while reading stays in the review list as it was. The script used for dictionary lookup can load its own dictionary or be newly developed. The code is JavaScript, the license is MIT, and the star count is 1616.
The limits are listed in the README. The GitHub source does not include an offline dictionary or English word inflection data, that is, a list of words whose form changes, such as plurals or tenses. The Anki integration also works only when Anki and AnkiConnect are already running, and although the repository was created on January 22, 2018, no public release can be confirmed. So this extension is closer to a tool that saves readers who already use Anki the effort of looking up unfamiliar words, and it does not yet answer environments that need an offline dictionary or where version-level management matters.