Editorial: fix normative statements

[jnurthen]

Closes HTML-AAM #590

1. Clarified passive voice statements - Converted statements like "MUST be exposed" to "User agents MUST expose" to explicitly identify the responsible party
2. Standardized normative language - Updated lowercase "should" and "may" to RFC 2119 compliant "SHOULD" and "MAY" for consistency
3. Fixed subject ambiguity - Added explicit subjects to statements that previously used passive voice, such as:

• "it MUST be exposed" → "User agents MUST expose"
• "MAY be treated" → "user agent MAY treat"
• "MAY be provided" → "Authors MAY provide"

4. Improved clarity - Enhanced descriptions of accessibility tree inclusion for media elements by restructuring text into bulleted lists
5. Corrected inconsistent terminology - Changed "browser" to "user agents" for consistency throughout the specification

[netlify]

✅ Deploy Preview for wai-aria ready!

Name	Link
🔨 Latest commit	df7cd05
🔍 Latest deploy log	https://app.netlify.com/projects/wai-aria/deploys/68b72254b5c2f40008596d84
😎 Deploy Preview	https://deploy-preview-2590--wai-aria.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[Copilot]

Pull Request Overview

This PR fixes normative statements in the HTML-AAM specification by clarifying the subject of requirements and standardizing the use of RFC 2119 keywords (MUST, SHOULD, MAY).

• Updated statements to explicitly specify "user agents" as the subject of requirements instead of using passive voice
• Standardized capitalization of RFC 2119 keywords for normative statements
• Improved clarity of technical requirements for accessibility API mappings