safari not rendering web page correctly

Is this on ↓: ?

iPhone 6, iOS 12

iOS 12 (2018–2019) has an older version browser (Safari 12), with somewhat different behavior. Support for some CSS properties came later.

… chrome, edge, Firefox which will use safari rendering engine I believe.

They do not. They each have their own rendering engine.

I’ve had the css chdcked and it’s correct

CSS Validator says: 2 issues found:

• Property “container-type”: “inline-size” ← Supported for Safari 16 or later/Chrome 105 or later/…

• Property “grid-template-rows”: “subgrid” ← Supported for Safari 16 or later/Chrome 117 or later/…

How can I force an update to my browsers?

You can’t. The OS version puts limits on the browser versions. Besides, you have no control over the devices and software versions of your visitors.

… don’t know how many users are or would be impacted

Safari 15 or earlier, Chrome 116 or earlier, etc. …

See the ‘Browser compatibility’ listing for each property on the developer.mozilla.org site [1] [2].