Improve accessibility

[stevenjoezhang]

PR Checklist

• The commit message follows guidelines for NexT.
• Tests for the changes was maked (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs in NexT website have been added / updated (for features).

PR Type

• Bugfix.
• Feature.
• Code style update (formatting, local variables).
• Refactoring (no functional changes, no api changes).
• Documentation.
• Translation.
• Other... Please describe:

What is the current behavior?

Issue resolved:

https://pagespeed.web.dev/report?url=https%3A%2F%2Ftheme-next.js.org%2F&form_factor=desktop

See also theme-next/hexo-theme-next#130

What is the new behavior?

• Link to demo site with this changes:
• Screenshots with this changes:

How to use?

In NexT _config.yml:

[coveralls]

Pull Request Test Coverage Report for Build 3494548607

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.614%

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Totals
Change from base Build 3477527997:	0.0%
Covered Lines:	394
Relevant Lines:	399

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💛 - Coveralls