Glossary · Web Development

Bootstrap

Bootstrap is a widely used front-end framework of prebuilt CSS components, a grid system and optional JavaScript for building responsive sites quickly.

Updated October 6, 2026

Bootstrap is an open-source front-end toolkit that provides ready-made CSS and JavaScript components, such as navbars, buttons, modals and forms, plus a responsive grid system. It was created at Twitter and released publicly in 2011.

Why it matters

Bootstrap lets small teams ship a consistent, responsive interface without designing every component. Its grid divides a row into 12 columns that rearrange at breakpoints, and its documentation covers accessibility basics for many widgets. Customization is done through Sass variables or CSS variables.

The trade-off is recognizability: untouched Bootstrap sites look alike, and loading the full stylesheet includes rules you never use.

Example

<div class="container">
  <div class="row">
    <div class="col-12 col-md-6">Left</div>
    <div class="col-12 col-md-6">Right</div>
  </div>
</div>

On phones each column fills the full width. From the md breakpoint upward, the two columns sit side by side at 50 percent each.

Common mistakes

  • Overriding styles with long selectors and !important instead of changing the Sass variables.
  • Loading the whole library, including JavaScript, when only the grid is used.
  • Nesting columns without wrapping them in a new .row, which breaks gutters.
  • Mixing Bootstrap versions; class names changed between major releases.

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