Center fixed div with dynamic width (CSS)

Expanded answer: Fixed positioning with a twist

To keep a div fixed and centered on screen, CSS transforms commingle with position: fixed;:

.fixed-center {
  position: fixed; /* Like a sticky note on your screen */
  top: 50%; /* Halfway from the top, pretty straightforward */
  left: 50%; /* Halfway from the left, pairing well with top: 50% */
  transform: translate(-50%, -50%); /* Now this one's tricky. It's like saying, get me half back in reverse from where you are! */
}
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<div class="fixed-center">Centered and Fixed Content</div>
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Herein, our .fixed-center div is always centered, horizontally and vertically, irrespective of content size dynamics.

Better Responsive Fluidity: Set max-width, play with viewport units

Construct a dynamically centered div while upkeeping constraints of max-width as shown:

.responsive-center {
  width: 90%; /* Allow the div to fill 90% of the container */
  max-width: 900px; /* But don't go overboard - 900px max! */
  position: fixed; /* Our old friend, helping us keep it pinned */
  left: 50%; /* Start mid, right? */
  transform: translateX(-50%); /* But slide back half the div's width to really nail the center */
}
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Melding methodologies for unflappable centering

Combine flex shorthands and CSS positioning for a dynamic, fail-safe centering solution:

1. Flexbox shorthands to maintain uniformity, and give browser prefix blues a miss.
2. Pair .fixed-center and max-height or min-height, ensuring dynamic content heights are kept in check.
3. Master calc functions, like 50vw and 50vh, for more precision with transform: translate;.

Corner-case considerations

Pinpoint special scenarios where the center might not hold:

• Transitioned views: div shifting smoothly? Add transitions to transform.
• Interactive elements: Dropdowns or modals messing your layout? Plan your centering accordingly.
• Cross-browser compatibility: Browser throwing a tantrum? Test your centering extensively.

Deciphering alternatives: Beyond the realm of fixed positioning

On occasion, fixed positioning is not what you seek:

• Scroll-friendly content: Roll with relative or absolute positioning.
• Overlaps abound: Watch out for z-index conflicts, opt for safer positioning.
• Responsive design: Fixed elements acting up on smaller devices? Flex that flexibility.

Ensuring cross-device compatibility

Make your centered div look spiffy on all devices:

• Utilize media queries to customize for various screen sizes and orientations.
• Experiment with different content lengths to ensure centering stays intact.
• Employ CSS variables for margins, sizes, et al., for easier adjustments as viewports change.