Features of the Opera web browser

This article details features of the Opera web browser.
Legacy features.
Many notable features, there were available in older versions of the Opera web browser, were removed when Opera switched from its legacy layout engine, Presto, to the Chromium-based Blink engine. Presto was introduced in 2003 (Opera 7) and was used until Opera 12.18, in 2016.
Compatibility.
Common compatibility problems are caused by websites not following standards or using methods for detecting the browser being used. To cope with outdated detection methods or poorly built websites, Opera enables users to change the information that is sent to websites to identify what kind of browser is being used—known as the user-agent. In previous years, Opera came preconfigured to partially "cloak" itself as Internet Explorer, but still included the word "Opera" in the user-agent information allowing the browser to be counted in web browser statistics. As websites modernized themselves and Opera 9 became more compatible with IE code, Opera began to use its own identification by default.
Later versions of Opera offered a limited method of cloaking allowing selection from a pre-defined range of options including Mozilla and Internet Explorer. If needed, Opera can mask completely as Internet Explorer or Mozilla, leaving out the reference to Opera in the UA string and JavaScript objects. Some sites test only for objects that are not present in Opera.
The version 8 of Opera introduced a further provision for dealing with faulty coding, by providing a set of scripts in "BrowserJS" that rewrites known broken pages as they are being opened. The closely related UserJS (similar to Mozilla's Greasemonkey), allows users to run their own code at various times in the processing of a page. These techniques have allowed many popular but incompatible sites to be used fully with Opera.
Opera periodically updates itself with the latest version of "BrowserJS" and "override_downloaded.ini" files to keep more sites working correctly in the browser, respectively, they serve to patch websites which would otherwise have issues to run on Opera and to cloak the UA string by default for some website domains.