Richard Nesbitt

Adds a fixed container with 3 buttons to the right hand side of your webpage. Allows users to increase, decrease, or reset font sizes for their optimal viewing experience.
This plugin uses localStorage to save the viewers preferences, and loads on every page so their font changes carry over as they go deeper into your site. As long as the users cache persists, their preferences will be saved for when they return.

Font Resize – Accessibility Compliance

Adds a text resize widget on the front end of your site.

Kybernetik Services

WP Cool Tools

Martin Stehle

This tiny plugin adds silently the viewport meta tag with zooming permission to allow your users to zoom in your website with mobile browsers.
If you find yourself in trouble not to be allowed to zoom with your favourite smartphone in your theme, you can take this tiny plugin just to get the shiny magic zooming with your smartphone in your theme.
There are themes which are “responsive”. But a few of them are cruel. They do not let you zoom in-to your theme.
Don’t be upset, don’t get in panic, don’t spend a lot of money. Allow mobile zooming definitely in your theme.
You just download this plugin and click to activate it. No options page, no settings. Just your theme.
This plugin does all the magic for you and your users zooming with your smartphone in your theme.
Instead, just adding a second viewport meta line in the header section of a page, I started with version 1.5.0 to add theme specific viewport replacement, if available. 
If you know theme which allows changes in the viewport by a filter, please write a comment in the <a href="https://wordpress.org/support/plugin/definitely-allow-mobile-zooming" rel="ugc">WordPress forum</a> and I will add the specific filter.
If you find yourself in trouble with this tiny plugin, don’t hesitate to ask for <a href="https://wordpress.org/support/plugin/definitely-allow-mobile-zooming" rel="ugc">support</a>.
Try it out on your free dummy site and <a href="https://demo.tastewp.com/definitely-allow-mobile-zooming" rel="nofollow ugc">click here</a>.
The plugin does not collect any personal data, so it is ready for EU General Data Protection Regulation (GDPR) compliance. In your theme.

Definitely allow mobile zooming

This tiny plugin adds silently the viewport meta tag with zooming permission to allow your users to zoom in your website with mobile browsers.

Font Resize – Accessibility Compliance vs Definitely allow mobile zooming