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@nicolaschevobbe @adamhotep I also had to look that up, but here's an example where a shadow DOM is used: In MDN's web dev playground, they want to isolate the CSS you write to only act on "your" HTML, not on the entire site. So they put it in a shadow root.
So $$ doesn't access the <h1> here, but $$$ does!
Ohh, the CSS pane helps you debug values of the "transform" property by showing you the box before and after the transformation!
That's all cool things I found in the Firefox DevTools today! Looking forward to check out the other tabs some other time.
It's kind of eye-opening for me how many convenient things a tool like that has. So far, when I opened the Toolbox, I usually quickly wanted to get something specific done, and didn't take the time to wander & explore…
What are your favorite built-in dev tools in Firefox? What did I miss in "Inspector" and "Console" today?
But also, I learned that the one-line mode tries to guess whether your command is incomplete when you press Enter, and then allows you to type a second line.
You can force a new line using Shift+Enter.
There's a built-in multi-line editing mode! Press Ctrl+B to turn it on. Seems useful to write longer expressions.
This mode shares its history with the regular one-line mode.
There's a couple more, like copy(), which copies its argument to your clipboard, or keys(), an abbreviation for Object.keys().
If you ever forget these console helpers, you can run :help to open the page of the documentation that explains them!
https://firefox-source-docs.mozilla.org/devtools-user/web_console/helpers/
There are more "Console Helpers" in #firefox:
$_ is a shortcut to the result of the last expression you executed, allowing you to do some neat step-by-step exploration.
And $0 refers to the currently-inspected element on the page.
Next, I looked at the Console.
I learned that it has some predefined helper functions! You can use `$(selector)` as an abbreviation for `document.querySelector(selector)`.
Similarly, `$$(selector)` is short for `document.querySelectorAll(selector)`!
Even though this reeks of jQuery, I think these are fantastic in this context and I might use them *a lot*!
As you start modifying the CSS rules, and arrive at something you like, you can switch over to the "Changes" tab to find a summary of what you changed!
You can also copy all of that into your clipboard, and then integrate it into the original CSS that you're working on!
Until now, I always made changes in my source CSS files directly, and used some auto-refresh tool to preview it. But this workflow of modifying it directly, and copying out the required changes might be a bit smoother!
In the Inspector, Ctrl+F jumps to the search box that allows you to find a tag or some content.
I was always annoyed that F3 doesn't go to the next result. Turns out that the shortcut for that is:
Enter!
If you click on the dashed box after a more general rule, you'll get all matching elements highlighted.
Here, I highlight all <p> elements of that toot.
In the CSS pane, there's this little dashed box icon after CSS rules. Click it to permanently highlight the matching elements on the page!
Here, I highlight a specific <p> element. The colors that are used correspond to the colors in the "Layout" tab, and show you the size of the margin, border, padding, and content area.
You can open the web console in a split view by pressing Esc!
This is the same console as in the "Console" tab. And this works in any tab, not just the Inspector! Super useful if you wanna see more things at once!
This is probably common knowledge, but I wasn't aware of it so far:
The sun & moon icons in the "Rules" tab allow you to quickly force the site into light mode/dark mode! 
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All elements with "display: flex" have a little "flex" pill icon next to them. If you click it, it will highlight the structure of the flexbox, and keep it highlighted as you interact with it!
All tree views in the DevTools have pretty good support for keyboard shortcuts!
You can use arrow up & down to highlight different elements, and arrow right & left to unfold/fold the elements.
You can find a full list of keyboard shortcuts here: https://firefox-source-docs.mozilla.org/devtools-user/keyboard_shortcuts/index.html
Let's start with the "Inspector", which shows you the tree of HTML elements.
You can directly summon it, and inspect a specific element on your current site by pressing Ctrl+Shift+C (Cmd+Opt+C on macOS), and then clicking your element.
This is similar to right-clicking the element, and pressing Q, but I like Ctrl+Shift+C a bit better because it will give you a live preview of which node you are picking!
Here's a thread of cool things I found exploring the #Firefox Developer Tools!
First, a really convenient thing: You can "pop out" the Toolbox into a separate window!
To do this, open the Toolbox (using F12), click on the ellipsis menu on the right, and select "Separate Window".
I like this much better than the default "docked" modes, especially when using a scrolling window manager like #niri!