When comparing Foundation vs Semantic UI, the Slant community recommends Foundation for most people. In the question“What is the best CSS framework?” Foundation is ranked 5th while Semantic UI is ranked 7th. The most important reason people chose Foundation is:
Foundation allows designing for multiple screen sizes simultaneously easily, meaning your content will always fit.
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Pros
Pro Responsive design philosophy
Foundation allows designing for multiple screen sizes simultaneously easily, meaning your content will always fit.
Pro Uses REMs instead of pixels
Foundation uses REMs instead of pixels, meaning you don't have to state an explicit height, width, padding, etc, for every device. Simply put, using REMs means you can just state font-size: 80%; and have the whole component (and its nested elements) shrink by 20%.
This is great for making your site mobile friendly. There is also a Sass function in Zurb that converts pixels to REMs so if you're used to thinking in pixels, you don't have to learn a different system.
Pro No style lock-in
Styles are purposefully undeveloped to encourage differentiation between different sites using Foundation.
Pro Block grid
Foundation has a feature called block grid. Block grid gives designers the power to divide the contents of an unordered list into a grid that is evenly spaced. Furthermore, Foundation also takes care of collapsing columns as well as removing gutters.

Pro Easy customization
Just by looking at the name, Foundation merely provides designers with a foundation of sorts on which they can build their design. It can be customized easily through SASS, a powerful CSS pre-processor or by overriding the default CSS styles.
Pro Easily extensible with a selection of add-ons
There's a variety of front-end templates, icon fonts, responsive table examples, SVG icons and stencils that help you quick-start or easily improve on your site.
Pro Support for off-canvas navigation
Foundation comes with an easy way of creating off-canvas menus.
Pro Uses Interchange to load responsive content
Foundation comes with Interchange, it makes use of media queries to load images responsively and create content that's suited to different browsers and devices.
Pro RTL support
Allows easily changing text direction.
<html class="no-js" lang="ar" dir="rtl">
Pro Built-in form validation
Foundation comes with Abide plugin, an HTML5 form validation library.
Pro Good mobile support
Foundation was one of the first frameworks to adopt a mobile-first philosophy. By focusing on mobile design first, Foundation makes designers think on what kind of content is important, relevant and interesting to the users without thinking too much on the space.
Pro It's semantic
Uses semantic class names for its styling, making it easier to grasp and understand even for beginners looking to jump right in.
Pro Load only the components you need
The whole Semantic-UI package is well organized, with every component neatly set up with it's own stylesheet and JavaScript file. This way you can load only the components you need for each page, minimizing the load time and file size.
Pro The elements offer a huge amount of customization, far beyond a framework like bootstrap
Pro Beautifully designed
Semantic UI has a futuristic and beautiful design. Many will satisfied with the design, especially when Semantic UI is used as a CSS framework.
Pro It'll have almost any UI component/element you may think of for your project
It includes tons of UI components that you may need for almost any type of project you may work on; And of course, you can always only pick what you need.
Pro Well documented
The documentation is easy to use, well written and has lots of examples each with their source codes.
Pro Easy to use
Just start code from the beginning of the journey, from first page of documentation.
Cons
Con Can be hard for beginners to grasp
Since Foundation is built to be customizable, it's default style may not be very appealing for most. While it's true that most production-ready websites shouldn't be using the default style of a css framework (they would all end up looking the same), this is even more true for Foundation.
Con Needs more pre-built components
Example would be a scroll-spy not only for one cell, but cell to cell.
Con Not UMD pattern in core
This problem will bring attention when used with Angular, React and other JS framework. It is important to know that they create app version of this framework.
Con Documentation is a bit better than average
Documentation could be written better and clearer, with many more example than they currently have. Sometimes hard to find solutions for detailed css problems.
Con Large file size
Packages are much bigger when comparing to Twitter Bootstrap or Zurb Foundation. Semantic UI is really extremely large and it would be better to use specific modules and components, rather than the whole thing.
Con Not maintained anymore
Use Fomantic-UI instead.
Con Buggy
Contains a lot of UI inaccuracies, like wrong positioning, cannot mix classes, etc.
Con Not for beginner developer/unfamiliar with Javascript
Many features in Semantic UI uses Javascript customization such as for Modal. This is unlike Bootstrap that can add Modal just with customizing the HTML attributes. Developers who plan to using Semantic UI must be familiar with Javascript or JQuery to get the most out of it.
Con Small number of classes
Once you wanna do something that is not mentioned in the doc - prepare to spend an hour, then give up and implement a custom "workaround".
