They haven't copied any content. Outline is a javascript app that changes the way your browser renders the website.
It's your browser contacting economist.com and fetching the webpage. The javascript app then changes how it is rendered by removing stuff most people don't want to see.
Outline is available as a browser extension, if you don't want to load it by visiting outline.com.
That's not true. It doesn't load the content through your browser, it loads it through its servers. They indeed copy content, and it's up to you to develop your own opinions as to the legality or ethicality of that. :)
It's your browser contacting economist.com and fetching the webpage. The javascript app then changes how it is rendered by removing stuff most people don't want to see.
Outline is available as a browser extension, if you don't want to load it by visiting outline.com.