Error combinator is obsolete?

Question

JavaScript

There is a .js file, how do I connect it to the .html file, an error is displayed in the console

/deep/ combinator is deprecated. See https://www.chromestatus.com/features/6750456638341120 for more details.

And the address file:///C:/Users/Gayan/Documents/App/index.html
changes to file:///C:/Users/Gayan/Documents/App/ null
but for example if I include a .js file everything works in codepen

Answer 1

This is not an error, but a warning.
The /deep/ combinator (and also ::shadow) was used to “fall through” into the shadow tree (Shadow DOM), now there are mixins and variables (CSS mixins, CSS variables), in connection with which /deep/ and : :shadow have been deprecated and will be permanently removed in the not too near future (around the beginning of 2017 tentatively, no exact date yet).
Since you essentially have two questions in one, here's the answer to the second part: The file:// protocol in many browsers has severe restrictions on what you can do with it. For web components to work properly, raise a web server and open a page from under it, and not just by opening a file in a browser. For simplicity, you can install the same Open Server that Alexander Taratin advised

Answer 2

Use open-server.ru
for local development.