Awesome

<img src="extension-folder/linter.png" height="50px"> in-browser-style-linter

Chrome extension / Firefox add-on

Do quick style checks on your page after it renders, without leaving your browser, and with minimal friction to setup (important for adoption):

1. Add settings.
2. Run in-browser. (Chrome/Firefox/IE)
3. See red buttons.

Or follow this demo: https://youtu.be/eK5jMvivitQ

Side step the worry of whether your style will be overridden, or whether you're accurately testing/simulating the computed styles; make use of the fact that the browser already renders the styles that the user sees.

STEP 1: To add a setting, edit this part of the JS code:

// Enter your desired settings here:
var settings = [
  {
    selector: "a", // CSS selector
    property: "color", // CSS property to check
    expectedValues: ["red", "rgb(88, 96, 105)"], // acceptable expected values of property
    // contains:true // OPTIONAL: boolean to say actual value can at least contain the expected value
    // innerHTML:'Some innerHTML text.' // OPTIONAL: you can be more specific than CSS selectors
  },
];

to something like this (try it on https://www.google.com):

// Enter your desired settings here:
var settings = [
  {
    s: "a",
    p: "font-family",
    v: "avenir",
    i: "About",
  },
  {
    s: "a.gb_d",
    p: "font-family",
    v: "avenir",
    c: true,
  },
  {
    s: ".gLFyf.gsfi",
    p: "color",
    v: ["#eee", "rgb(88, 96, 105)", "#3e3e3e"],
  },
];

(For details, see more info.)

STEP 2: Set up your browser to run the JS code:

• Chrome extension: In-Browser Style Linter
• Chrome: snippets
• Firefox add-on: In-Browser Style Linter
• Firefox: console in Multi-line mode
• Internet Explorer: F12 > Console > paste the whole code into the terminal (paste after the ">" symbol on the bottom) > Ctrl+Enter (or hit run)

STEP 3: You'll see ugly red buttons, like this:

Hovering over the error button shows the expected and actual values. See this demo: https://youtu.be/eK5jMvivitQ

More info: (click to expand)

Minimal required info:

var settings = [
  {
    selector: "a", // a CSS selector like 'div span a:hover'
    property: "color", // a CSS property
    value: "red", // the expected value after page render
  },
];

All parameters have short forms to let you save on keystrokes. Here's an equivalent to the example above:

var settings = [
  {
    s: "a", // s is for selector
    p: "color", // p is for property
    v: "red", // v (or ev) is for expected value
  },
];

To relax the matching of the property value to simply "contain" the expected value, set the optional contains parameter to true:

var settings = [
  {
    selector: "a",
    property: "background",
    value: "#333",
    contains: true, // would not flag '#333 url("img_tree.gif") no-repeat fixed center' as error
  },
];

All parameters have short forms to let you save on keystrokes. Here's an equivalent to the example above:

var settings = [
  {
    s: "a",
    p: "background",
    v: "#333",
    c: true, // would not flag '#333 url("img_tree.gif") no-repeat fixed center' as error
  },
];

To specify several allowable expected values, use an array:

var settings = [
  {
    selector: "a",
    property: "color",
    value: ["red", "rgb(88, 96, 105)"],
  },
];

This is also compatible with the "contains" option (see above).

To specify elements that have a specific innerHTML (in addition to the CSS selector), set the optional parameter value:

var settings = [
  {
    selector: "a",
    property: "color",
    innerHTML: "Some innerHTML text.", // check the color of <a> tags with this innerHTML
    value: "rgb(88, 96, 105)",
  },
];

All parameters have short forms to let you save on keystrokes. Here's an equivalent to the example above:

var settings = [
  {
    s: "a",
    p: "color",
    i: "Some innerHTML text.", // check the color of <a> tags with this innerHTML
    v: "rgb(88, 96, 105)",
  },
];

You can use variables to update properties in one place instead of updating the whole settings array. For example:

var myColour = 'blue'; // you edit the value here, just one place

var settings = [
    {
        s:'button.btn.btn-info',
        p:'background',
        v:myColour
    },
    ...
    {
        s:'a.some-fancy-button',
        p:'background',
        v:myColour
    },
    ...
    {
        s:'label.consistent-styling-ftw'
        p:'background',
        v:myColour
    }
];

And avoid situations like this:

var settings = [
    {
        s:'button.btn.btn-info',
        p:'background',
        v:'blue' // edit here
    },
    ...
    {
        s:'a.some-fancy-button',
        p:'background',
        v:'blue' // and here
    },
    ...
    {
        s:'label.consistent-styling-ftw'
        p:'background',
        v:'lightblue' // oops I forgot, where else do I have to change this?
    }
];

Want to understand how the code works?

In my mind, the key line of code is this:

var elements = document.querySelectorAll(
  selector + ":not(.in-browser-linter-button)"
);

If you want more details:

I recommend you start with reading the standalone snippet: https://github.com/hchiam/in-browser-style-linter/blob/master/linter.js

History alert: Pure JavaScript. No jQuery. This was originally just a snippet I'd copy and paste into Chrome DevTools.

Conceptual data flow in the extension-folder folder:

manifest.json -> popup.html (the settings popup) -> popup.js -> main.js -> (the summary popup)

Like this project?

A Chrome extension template repo: chrome-extension-template

Other Chrome extensions:

A Chrome extension project I helped tutor and contribute to: Habit-Tracker-Extension

A tool to quickly search links and menus (even if collapsed): quick-menu-search

Another Chrome extension (note: very experimental): select-hover-search

Another Chrome extension (note: very experimental): in-browser-test-automator

Firefox add-ons:

Console Log Element

Check All Scripts with URLVoid

Want to make a linter in VS Code instead?

Custom VS Code linter template repo: custom-vscode-linter