Since the inception of responsive web design, one of the things we started doing is adding the viewport meta tag to the head of our HTML. But sometimes people use it to disable zooming, and I wish they didn't.
When using the viewport meta tag, you should never disable zooming. It's bad for accessibility, and people with impaired vision will have a hard time reading your content. But it's not just them; disabling zooming is bad for everyone!
The following is probably all you need. It means that the browser should render the width of the page at the width of its own screen
(thanks to CSS-Tricks for the explanation):
<meta name="viewport" content="width=device-width, initial-scale=1">Ideally your typography should be nice and big anyway, so your users won't need to zoom. But it's nice to give people the option.
Avoid the minimum-scale, maximum-scale, and user-scalable properties.
Some browsers, such as Firefox, offer colour vision simulation in their accessibility tools. This lets you see how a person with impaired vision might perceive your content. I recommend trying it. It's only an approximation, but it might help you feel more empathy for those with visual impairments.