What is the viewport meta tag, and do you still need it?
Yes, and it is not optional. It is the one line that tells a phone to render your page at the phone's width. Without it, a mobile browser assumes your site was built for a desktop and shows a shrunken, zoomed-out version of it.
What it actually is
Mobile browsers inherited a compatibility behaviour from an era when almost no site was designed for phones: pretend to be a desktop screen, render the page wide, and scale the result down so it fits. The viewport tag switches that off and says this page knows what a phone is. Everything responsive in your design depends on it being there.
Why it matters
Google indexes the mobile version of your site as the primary version, so the phone rendering is not a secondary concern — it is the one that counts. A page missing this tag is not slightly worse on mobile, it is effectively unusable: text too small to read, buttons too small to hit, and a visitor who leaves within seconds. That behaviour is measured, and it feeds back into how the page is judged.
What good looks like
Every page declaring that it should be rendered at the device's own width at normal zoom, and doing so without preventing people from zooming in. That last part matters — some sites disable zoom to keep a layout tidy, which breaks the page for anyone who needs to magnify text to read it.
Being honest about it
This is close to non-negotiable and it is also nearly always already correct, because every modern theme and site builder includes it by default. When we do find it missing, it is a strong signal that something unusual is going on — a hand-built page, a legacy template, or a landing-page tool bypassing the site's normal layout.
Where most sites go wrong
It is rarely absent site-wide. The realistic failure is a single page escaping the normal template — a campaign landing page, an embedded application, a page built by a different tool — where the rest of the site is fine and one commercially important page is not. The other real problem is zoom being disabled, which passes a naive check while making the page inaccessible to a substantial number of people.