This project provides three different linters: one for HTML, one for Sass (.scss-files to be specific) and one for JavaScript.
All those linters can be configured in their set of rules, and they can be globally deactivated in config.js, section lint.
The linter used for your HTML-Code is htmlhint. This project comes with a pre-configured set of rules to enforce nice HTML-styling, those rules and their current setting can be found in the file .htmlhintrc, description for the rules can be found on github.
If you use a CMS or any other tool that creates your HTML-Output, it sometimes makes more sense to just deactivate linting for HTML or change some of the default rules. But please always check your code for errors with tools such as the W3C Validator to keep yourself sane. Valid HTML (at least regarding nesting and quotes) can save you a lot of headaches when styling your website.
The linter used for JavaScript is eslint, and this project comes with a pre-configured (kinda restrictive) set of rules, to enforce a consistent and sane code style. You can find all current rules and the settings for them in the file .eslintrc.
The predefined .eslintrc.yml-file sets all rules you can find in the eslint documentatoin, in the same order for easy reference.
Eslint is very customizable, you can write your own plugins, but for starters the default built-in rules are more than enough.
Since we use ES6 in this project, eslint is configured to warn you on some things, that can be written better with ES6 (such as arrow-functions or let vs. var). If you don’t want to use any ES6-features at all, you can just set all those rules to zero, to deactivate them.
For linting our Sass we use sass-lint. Configuration for sass-lint can be found in .sass-lint.yaml, and is pre-configured to be quite strict. All avaiable rules for sass-lint can be found here. By default anything in the folder ./src/scss/vendor will not be linted, so you can add third-party stylesheets easily.
As always: try to keep to the strict defaults and only in edge-cases change the default rules and allow exceptions (such as !important).