When comparing Bootstrap vs Flutter, the Slant community recommends Bootstrap for most people. In the question“What is the best CSS framework?” Bootstrap is ranked 2nd while Flutter is ranked 25th. The most important reason people chose Bootstrap is:
The grid layout with the predefined CSS elements and JavaScript components make it easier to have consistency across different browser versions and even different devices.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Consistency across browsers
The grid layout with the predefined CSS elements and JavaScript components make it easier to have consistency across different browser versions and even different devices.
Pro Responsive design philosophy
Bootstrap is developed to be instantly compatible with all sizes of screens, so you don't have to worry about which device the user is accessing your site from. Yet if you prefer, you can disable responsiveness of Bootstrap.
Pro Well documented
Documentation is thorough, well organized and full of live examples and templates ready for use. Every component and every part of the framework is explained and covered in depth.

Pro Custom builds
If you don't want to download the full Bootstrap framework. Custom builds of Bootstrap can be created, including only the desired CSS, CSS components, and JavaScript components. This can be done directly from the Bootstrap website by simply choosing what components to download.
Pro Designed to get a site running quickly
Bootstrap is designed to get a site up and running quickly. Each of it's components is pre-configured to help with getting a site up and running quickly.
Pro Great community
Bootstrap is very popular and has a large community. As a result of this it is much easier to find help with anything you might need. This also gives you a treasure trove of prebuilt components to use and add to your site.
Pro Supports responsive embeds
Allows easily adding responsiveness to <iframe>
, <embed>
, and <object>
elements.
Pro Easily make professional looking websites
Bootstrap makes it easy to learn how to make professional looking websites. It can even make code junkies semi-enjoy design.
Pro Customizable
Bootstrap can be customized in a variety of ways. Either by overriding the default CSS styles with new CSS styles or by editing the .scss
Bootstrap files.
Pro A wide variety of themes available
To help you avoid the "Bootstrap look", there are many resources that provide a great selection of themes and templates for Bootstrap.
Pro One of the most popular ones
Ranked on GitHub as the most starred CSS repository.
Pro Open source
Pro Great developer tools debug/hotreload/analyser
Pro Based on Dart language
Dart is a Java like language, easy to learn and startup fast for millions of Java developers.
Pro A single codebase for iOS AND Android
Pro Fast
The developer's goal is to allow people to make apps running at 120 FPS.
Cons
Con Large file size out of the box
Bootstrap has an out-of-the box filesize of ~276K, which is pretty large considering it's just CSS. Most of those styles aren't even used in 90% of web pages built with Bootstrap. By only including the required styles it can be trimmed by 70%-75%.
Con Too many classes
Bootstrap's over-reliance on HTML classes for styling can get very messy very quickly. There's also an overabundance of DOM elements which have a lot of classes and are more often than not nested inside DOM elements with even more classes. This gets problematic down the line because the maintainability of the project gets harder when the project starts to get large.
Con Websites can start to look the same
If the initial Bootstrap colors and styles are not changed or edited, different websites start looking the same even if they have nothing to do with each other and they are made by different developers.
Con Hard to customise
It's quite hard to customise Bootstrap using pre-processors like SASS and LESS, the only real flexibility is with typography and colours.
Con Large font sizes
Con Still in development
According to the website, Flutter is still in its early stages of development.
Con Based on Dart language
Dart is a Java like language, easy to learn and startup fast for millions of Java developers. BUT if you have to learn it ... it's a con.
Con Dart is unpopular and never gained serious community traction like Kotlin or Java
Con Does not support 32-bit iOS devices
If you plan on targeting iPhone 5, 5C or earlier, you can forget about Flutter.
Con Google has a bad history with product loyalty
