Mobile viewport is not set

On every live page we look for a single element: <meta name="viewport">. If it is not in the HTML, the finding is raised. No threshold, no heuristic: it is there or it is not.

What the check does not do:

• It does not judge the value. content="width=1200" passes just like a correct width=device-width, even though the first fixes nothing about mobile rendering.
• It does not test how the page actually looks on a phone. We measure neither content width, nor overflowing tables, nor font size. The viewport tag is a necessary condition for responsive rendering, not a guarantee of it.
• It does not flag user-scalable=no or a fixed maximum-scale, which forbid the user from zooming. That is an accessibility defect this check says nothing about.
• It looks at neither CSS media queries nor whether you run a separate mobile site on another URL.

The finding is page-level; severity is warning.

Here we have to be especially precise, because more myths circulate about mobile rendering than about anything else in SEO.

The viewport tag has no effect whatsoever on what a language model reads off your page. It is an instruction for rendering in a browser; the text in the HTML is identical with it and without it. Anything reading the delivered HTML (which, by measurement, is what most AI crawlers do, see ACC-03) cannot tell the difference.

Nor is it a condition for being indexed. Google states so in its own documentation:

While it's not required to have a mobile version of your pages to have your content included in Google's Search results, it is very strongly recommended.Google Search Central, Mobile-first indexing

What Google does say unambiguously, and what matters more to you: it indexes the mobile version. It crawls content with the smartphone agent and decides indexing and ranking from that. And it immediately adds a requirement that matters far more than the viewport tag: the mobile site must contain the same content as the desktop site. When there is less content on mobile, that lesser content is what Google works with.

So where is the value of this finding? With people. Without a viewport, a phone browser renders the page at desktop width and scales the whole thing down. The visitor gets unreadable type and has to pinch to zoom. That is a real and measurable problem; it is just a problem of usability, not of readability for AI.

The practical conclusion: it is a one-line fix, so make it. But do not expect it to move your visibility in AI answers. And if anyone promises you that, it is exactly the kind of claim this manual exists for.

This single line belongs in the <head> of every page. Nothing more, nothing extra:

<meta name="viewport" content="width=device-width, initial-scale=1">

What should not be in it:

• user-scalable=no: it forbids zooming. For a person who needs to see the page, that is a serious barrier, and modern browsers often ignore it anyway.
• maximum-scale=1: the same thing in other words.
• A fixed width (width=1024). You would be telling the browser to pretend it is a tablet when it is a phone.

The tag alone does not make a site responsive, though. Add it to a page whose layout assumes a fixed width and you get a page that overflows sideways on a phone, often worse than a scaled-down desktop view. After adding it, open the page on a phone or in your browser's device mode and check three things:

1. Nothing overflows horizontally. The usual culprits are tables, long URLs in body text, images without max-width: 100%, and preformatted code.
2. Text is readable without zooming. Around 16 px for body copy.
3. Buttons and links can be hit with a thumb. Links stacked 12 px apart are unusable on a phone.

And one more thing Google calls out as more important than any of the above: if you run a separate mobile site, it must carry the same content as the desktop one. A trimmed mobile version means Google (and with it the models that draw on its index) knows only the trimmed one.