While the popularised term of "mobilegeddon" might make you reminisce about a Michael Bay attempt at filmmaking (Armageddon was 17 years ago now), perhaps mercifully they're unrelated.

Indeed this dramatic term refers to Google's latest search engine update which is said to heavily weight favour towards mobile-accessible content.  Which is logical, given the notion of the ubiquitous web where all content is accessible at all times irrespective of how it is accessed, but there's a very valid question lurking behind those memories of Ben Affleck.

There are many ways of approaching making your content suitable for the web, which one is mobilegeddon going to favour?  Why?

It's perhaps worth reviewing what the accepted approaches are here:

3 valid approaches, but which will fare the best in mobilegeddon?  I think there's a very solid argument for mobile-first being favourable, and here's why.

Where the responsive methodolgy approaches accommodating mobile platforms by re-flowing and scaling assets accordingly, the focus remains on the desktop/tablet.  Content is stacked, elements are removed and the end product can sometimes result in a slightly clumsy information architecture.  If a site was designed to have key signposts in the right-rail, you often find these are stacked beneath the main site content when handled responsively.  This isn't necessarily a problem, but it could be.

Adaptive is the other side to the same coin, where layouts are specifically served for devices rather than merely desktop views flowed into acceptable formats.  Crucially it still leverages the same core content while affording you the freedom to serve different designs.  Principally it's a similar approach, though.

Where mobile-first is perhaps preferable is because of the core design principals it enforces.  It forces you to strip a website to its bare essentials, with questions like:

It forces you to distill a complex set of requirements down into a simple list of objectives.  Against these you can approach your information achitecture, signpost methodology and aesthetic layers with directness not found in other approaches.  Higher resolutions will then have enhancements augmenting the experience, but nothing "core" is added because it existed at the lowest break-point.

Why would this help me in mobilegeddon?

SEO is as much about logical page structure as it is about keyword density and other traditional concepts.  Heading strategy and the location of key content matters and these are far easier to get right in a mobile-first methodology.  If you get your core content structured correctly, written without being diluted by concepts mobile has no space for, the positive ripple-effect upwards through the higher resolutions is pretty logical.

If Google is assessing the "worth" of your page through mobile eyes, you could do far worse than ensuring you start there.  Granted, they've only stated that they're favouring sites that have mobile compatibility (irrespective of methodology employed) but there are zero downsides to approaching things mobile-first.  It's worth stating that they're not mutually exclusive either, you can approach things in a mobile-first manner and deliver it responsively, it just means the direction you approach it from is reversed.

Focus on delivering the best, most efficient and most effective experience with the least clutter and augment from there.  It means 100% of your users are getting the message you intended in the way you intended it, which is not only better for mobile SEO, it's just better in general.