Does Mobile-First Index Google update require Mobile-First CSS?

[Zivush]

Google rolled out an update called Mobile-First index to focus on mobile search.
My question:
Does this Algorithm change require from website not only being optimized with a simple responsive design, but also using responsive design applying Mobile-First CSS approach (as opposed to using Desktop First) ?
1. Mobile first CSS is much efficient IF Google made a decision to focus on Mobile search.
2. In Desktop First CSS you always overriding a lot of CSS (lots of processes = higher latency = slower mobile version. Therefore, I assume that it would be wise to take the Mobile First CSS approach, but I am still not 100% sure.

Any evidences?
Please advise

[Robert Charlton]

Mod's note: Zivush, can you clarify what you mean by "mobile-first CSS"? Your title was much longer than it is now, and the system clipped it off at 70 characters, so I wasn't exactly sure what your question was going to be, but I'm guessing from the post that you were asking about "mobile-first CSS".

There are, however, only two other uses of that phrase on the web, and I've never come across it before... so it's not clear what you're asking. Please let us know. Thanks....

[stever]

Possibly meaning AMP CSS restrictions on general site?

[Zivush]

Yes. I am speaking about mobile-first CSS.
Mobile First CSS means designing for mobile before designing for desktop or any other device.

Using a mobile-first layout approach with CSS, you serve the basic layout styles and minimal amount of code to style a page for a small, mobile device.

If you design for desktop first you have to UNDO/RESET a lot of the desktop styling to rearrange things for mobile.

If you start with mobile first you enhance the design as and when the screen size requires.
Moreover, there is a pretty good chance that the majority of users these days are on mobile devices. It makes sense to design the default around them (mobile) rather than the desktop users.
My question is : Might be that Google want to adopt this approach when rolling out the mobile first update ?

[keyplyr]

Design any way you wish. Nothing is"required" to be indexed in the Mobile-first index. Google will include your site even if it's desktop only.

However, for better ranking, put more consideration into those aspects specific to mobile: speed, readabile text, navigation, etc.

Personally, nowadays I design pages for mobile and through a responsive layout, then add a few additional components for the wider screen of desktop. Content volume is about the same.