Add "Outdated Browser" / Accessibility notice and basic a11y improvements

[Puru-Jangid]

• PR for this issue must follow the PR template
• This is a bug
• This is a feature request

For Feature request

• Explain the working of new feature.
  Add a small unobtrusive banner or modal that warns users on very old browsers and provides link(s) to update. Also implement basic accessibility improvements: semantic landmarks (<header>, <main>, <footer>), alt text for images, and keyboard focus styles.

• How will implementing this help this repo?

  • Improves UX for visitors on older browsers and increases accessibility compliance.
  • Reduces support questions and ensures content is usable for more people (including screen reader users).
  • Shows professional quality and attention to web standards.

• Relevant examples and resources

  • Simple browser-check snippet: feature-detection and show banner if document.querySelector or flexbox not supported.
  • Accessibility checklist: add alt attributes to images, use ARIA roles where needed, ensure contrast ratios are OK, and include :focus CSS styles.
  • Files to change: index.html, styles.css, optionally add a11y.js or small script for the banner.