How do you ensure correct page rendering in old or…

Using Progressive Enhancement: Start with a basic, working functionality for all browsers, then add more advanced features using JavaScript and CSS for modern browsers.

Applying Feature Detection: Instead of detecting the browser via user agent, use modernizr or custom code to check support for specific CSS properties or APIs.

// Checking support for MutationObserver
if ('MutationObserver' in window) {
  // Use MutationObserver
} else {
  // Use fallback (e.g., old EventListener)
}

Polyfills and Transpilers: Use babel to transpile modern JavaScript (ES6+) into an older standard (ES5) compatible with older browsers. For missing JS features, use polyfills (e.g., core-js). For CSS, use postcss with autoprefixer.

CSS Fallbacks: For advanced CSS features (e.g., with Grid or Flexbox), provide fallback options using float or inline-block.

/* CSS Grid with fallback for older browsers */
.container {
  display: block; /* Fallback */
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
  gap: 20px;
}

Minimizing JavaScript Dependency: Implement critical functionality using HTML and CSS as much as possible so that the site remains operational even if JS is disabled.
Testing: Regularly test the site in various old browsers (IE11, old Edge versions) using BrowserStack or local virtual machines.
Limitations: Discuss with the client which browsers should be fully supported and for which limited functionality is acceptable. Define the minimum support threshold.
Providing Alternative Resources: Use <picture> or <video> tags with different formats and sources for images or videos so the browser can choose the suitable one.
```
<picture>
  <source srcset="image.webp" type="image/webp">
  <img src="image.jpg" alt="Image description">
</picture>
```

How do you ensure correct page rendering in old or limited browsers?