Your website's framework may not be ADA-compliant!
One of the WCAG's requirements for ADA compliance is that website users must be able to manually increase the size of all content. By default, web browsers facilitate this with Control-plus (for Windows) or Command-plus (for Mac). However, some website frameworks, templates, and CMS platforms disable this functionality!
If you have a website, go to it right now and try to increase the text size: click somewhere in the blank space of the website (to make sure that area is in focus, rather than the browser interface, or another app that's open) and then press Control-plus (Windows) or Command-plus (Mac).
Did it work? Text got bigger? Then you're good!
Did it not work? Then your website's code is actively preventing this required functionality, and your site is not ADA-compliant.
How can one correct this?
If you're lucky, it may be a setting in your website's admin area (aka dashboard) that can simply be turned on or off; this would be unusual, but it's a possibility. Or, a web developer may be able to manually edit the code of your framework, template, or CMS platform to unblock this functionality. This might entail an easy edit of a few lines of code, or it could turn out to be a real nightmare and require a partial or complete overhaul of your site. There's no way to know until the dev can get in there and do some digging.
Another option, of course, is to switch your site over to a different framework, template, or CMS platform that does allow this functionality. It's possible to search specifically for a platform that's advertised as ADA-compliant, and that should cover this.
These options may ultimately be expensive, but ask yourself: is it better to spend the money to address this problem asap, or risk an ADA lawsuit down the line? And keep in mind, an ADA lawsuit may come with the added baggage of some very bad PR for your company or brand.
Why on earth would some website platforms block this content-zooming functionality? It's honestly hard to justify. It may be that the developer wanted to retain complete control over how content is viewed. It might even be a simple mistake or oversight; some left-over and ignored code that got picked up from something else. I really can't think of a concrete reason why a website owner should want to block this built-in browser functionality when it exists specifically to help people consume your content more easily. But we see it all the time.
If you didn't know, now you know!
– Manning
Questions/comments? Feel free to contact me at manning@manningkrull.com. I update these articles pretty frequently — best practices evolve over time as the world of digital quickly changes, and I always welcome insights from others.