Bootstrap 5 UI

Production patterns for building responsive, accessible web UIs with Bootstrap 5.3 in HTML and ASP.NET Core Razor Pages/Views. Target Bootstrap 5.3.x (current CDN: 5.3.8). Bootstrap 5 dropped jQuery dependency entirely.

Setup in ASP.NET Core

CDN (Quick Start)

In Layout.cshtml or Host.cshtml:

<!-- In <head> -->
<link href="https://cdn.jsdelivr.net/npm/[email protected]/dist/css/bootstrap.min.css"
      rel="stylesheet"
      integrity="sha384-sRIl4kxILFvY47J16cr9ZwB07vP4J8+LH7qKQnuqkuIAvNWLzeN8tE5YBujZqJLB"
      crossorigin="anonymous">

<!-- Before closing </body> -->
<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/js/bootstrap.bundle.min.js"
        integrity="sha384-FKyoEForCGlyvwx9Hj09JcYn3nv7wiPVlz7YYwJrWVcXK/BmnVDxM+D2scQbITxI"
        crossorigin="anonymous"></script>

LibMan (Recommended for Production)

// libman.json
{
  "version": "1.0",
  "defaultProvider": "jsdelivr",
  "libraries": [
    {
      "library": "[email protected]",
      "destination": "wwwroot/lib/bootstrap/"
    }
  ]
}

Reference locally in layout:

<link rel="stylesheet" href="~/lib/bootstrap/dist/css/bootstrap.min.css" />
<script src="~/lib/bootstrap/dist/js/bootstrap.bundle.min.js"></script>

Required Globals

Always include in <head>:

<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">

Grid System

Bootstrap uses a 12-column flexbox grid with 6 responsive breakpoints:

Breakpoint	Prefix	Min-width
Extra small	`.col-`	<576px
Small	`.col-sm-`	>=576px
Medium	`.col-md-`	>=768px
Large	`.col-lg-`	>=992px
Extra large	`.col-xl-`	>=1200px
XXL	`.col-xxl-`	>=1400px

Core Pattern

<div class="container">
  <div class="row">
    <div class="col-md-8">Main content</div>
    <div class="col-md-4">Sidebar</div>
  </div>
</div>

Key Rules

Always wrap columns in .row inside a .container (or .container-fluid for full-width)
Columns must be direct children of rows
Use .g- classes for gutters (.g-0 removes gutters, .gx- horizontal, .gy-* vertical)
Use .row-cols-* on the row for uniform column counts: <div class="row row-cols-1 row-cols-md-3 g-4">
Mix breakpoints: <div class="col-6 col-md-4 col-lg-3"> — stacks to 2-up on xs, 3-up on md, 4-up on lg

Container Types

.container — responsive fixed-width
.container-fluid — full-width always
.container-{breakpoint} — fluid until breakpoint, then fixed

Spacing Utilities

Format: {property}{sides}-{breakpoint}-{size}

Property: m (margin), p (padding) Sides: t top, b bottom, s start(left), e end(right), x horizontal, y vertical, blank = all Size: 0=0, 1=0.25rem, 2=0.5rem, 3=1rem, 4=1.5rem, 5=3rem, auto

<div class="mt-3 mb-4 px-2">Spaced element</div>
<div class="mx-auto" style="width: 200px;">Centered block</div>
<div class="py-md-5">Padding only on md+</div>

Color Modes (Dark/Light)

Bootstrap 5.3 supports data-bs-theme attribute for dark mode:

<!-- Global dark mode -->
<html lang="en" data-bs-theme="dark">

<!-- Per-component -->
<div class="card" data-bs-theme="dark">...</div>
<nav class="navbar bg-dark" data-bs-theme="dark">...</nav>

Use semantic color classes that adapt to color mode:

bg-body, bg-body-secondary, bg-body-tertiary — adapt to theme
text-body, text-body-secondary, text-body-emphasis — adapt to theme
border-body — adapts to theme

Avoid hardcoded colors. Use bg-body-tertiary instead of bg-light for theme-aware backgrounds.

Common Component Patterns

Navbar

<nav class="navbar navbar-expand-lg bg-body-tertiary">
  <div class="container-fluid">
    <a class="navbar-brand" href="#">Brand</a>
    <button class="navbar-toggler" type="button"
            data-bs-toggle="collapse" data-bs-target="#mainNav"
            aria-controls="mainNav" aria-expanded="false"
            aria-label="Toggle navigation">
      <span class="navbar-toggler-icon"></span>
    </button>
    <div class="collapse navbar-collapse" id="mainNav">
      <ul class="navbar-nav me-auto mb-2 mb-lg-0">
        <li class="nav-item">
          <a class="nav-link active" aria-current="page" href="#">Home</a>
        </li>
        <li class="nav-item">
          <a class="nav-link" href="#">About</a>
        </li>
      </ul>
    </div>
  </div>
</nav>

Cards in Grid

<div class="row row-cols-1 row-cols-md-2 row-cols-lg-3 g-4">
  <div class="col">
    <div class="card h-100">
      <img src="..." class="card-img-top" alt="...">
      <div class="card-body">
        <h5 class="card-title">Title</h5>
        <p class="card-text">Description text.</p>
        <a href="#" class="btn btn-primary">Action</a>
      </div>
    </div>
  </div>
  <!-- repeat -->
</div>

Forms

<form method="post">
  <div class="mb-3">
    <label for="email" class="form-label">Email</label>
    <input type="email" class="form-control" id="email">
  </div>
  <div class="mb-3">
    <label for="password" class="form-label">Password</label>
    <input type="password" class="form-control" id="password">
  </div>
  <div class="mb-3 form-check">
    <input type="checkbox" class="form-check-input" id="remember">
    <label class="form-check-label" for="remember">Remember me</label>
  </div>
  <button type="submit" class="btn btn-primary">Sign in</button>
</form>

Floating Labels

<div class="form-floating mb-3">
  <input type="email" class="form-control" id="floatEmail" placeholder="[email protected]">
  <label for="floatEmail">Email address</label>
</div>

Responsive Table

<div class="table-responsive">
  <table class="table table-striped table-hover">
    <thead class="table-dark">
      <tr><th>Name</th><th>Email</th><th>Actions</th></tr>
    </thead>
    <tbody>
      <tr><td>John</td><td>[email protected]</td><td><a class="btn btn-sm btn-outline-primary">Edit</a></td></tr>
    </tbody>
  </table>
</div>

Razor Pages Integration

For detailed ASP.NET Core Razor Pages integration patterns including tag helpers, validation, layouts, and partial views, see [references/razor-integration.md](references/razor-integration.md).

Key points:

Use asp-for, asp-page, asp-action tag helpers with Bootstrap classes
Combine asp-validation-for with .invalid-feedback / .is-invalid classes
Place Bootstrap validation classes via jQuery Validation Unobtrusive or custom JS
Use @section Scripts for page-specific scripts in layout

Component Reference

For complete component patterns (modals, alerts, toasts, accordion, pagination, breadcrumbs, badges, offcanvas, spinners, progress bars), see [references/components.md](references/components.md).

Utility Classes Quick Reference

Display

d-none, d-block, d-flex, d-grid, d-inline, d-inline-block Responsive: d-md-none, d-lg-flex

Flexbox

justify-content-{start|center|end|between|around|evenly} align-items-{start|center|end|stretch} flex-{row|column}, flex-wrap, flex-grow-1

Text

text-{start|center|end}, text-{lowercase|uppercase|capitalize} fw-{bold|semibold|normal|light}, fs-{1-6} text-truncate, text-nowrap, text-break

Colors

text-{primary|secondary|success|danger|warning|info|light|dark} bg-{primary|secondary|success|danger|warning|info|light|dark} text-bg-{color} — sets matching text+bg together

Borders & Shadows

border, border-{top|bottom|start|end}, border-{color}, border-{1-5} rounded, rounded-{0-5|circle|pill} shadow, shadow-sm, shadow-lg, shadow-none

Sizing

w-{25|50|75|100|auto}, h-{25|50|75|100|auto} mw-100, mh-100, vw-100, vh-100 min-vw-100, min-vh-100

Position

position-{static|relative|absolute|fixed|sticky} top-{0|50|100}, start-{0|50|100}, translate-middle

Accessibility Requirements

Always include aria-label on icon-only buttons and togglers
Use aria-current="page" on active nav links
Use aria-expanded on collapse/dropdown triggers
Provide alt text on images; use .visually-hidden for screen-reader-only text
Ensure color contrast meets WCAG 2.1 AA (4.5:1 for text, 3:1 for large text)
Use semantic HTML: <nav>, <main>, <header>, <footer>, <section>, <article>
Never remove :focus styles; use Bootstrap's .focus-ring utility for custom focus

Anti-Patterns

Using col-* without a parent .row — breaks grid alignment
Nesting containers — only nest rows, never containers inside containers
Using <br> and inline styles for spacing — use spacing utilities (mt-3, mb-4)
Hardcoding bg-light/bg-dark — use bg-body-tertiary for theme-aware backgrounds
Forgetting navbar-expand-{breakpoint} — navbar won't collapse responsively
Mixing Bootstrap 4 classes (e.g., ml-3, mr-3) — use ms-3, me-3 (start/end logical properties)
Omitting type="button" on non-submit buttons in forms — causes unintended form submission
Using <a> for actions without href — use <button> for actions, <a> for navigation
Skipping .form-label on form inputs — hurts accessibility
Not wrapping tables in .table-responsive — causes horizontal overflow on mobile

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