FOLLOW-UP: Document lvt-preserve-attrs class footgun in user-facing docs (from PR #72)

[adnaan]

Context

This follow-up task was identified during the review of PR #72.

Source PR: #72
PR Title: feat: lvt-preserve attributes, navigate SPA nav, DOMParser script fix
Suggested by: @claude[bot] (flagged across multiple review rounds)

Task Description

lvt-preserve-attrs only protects attributes that the server template does not set. This is a subtle but important constraint that is easy to misuse:

• If the server template sets class="card" and client JS later adds card open via classList.add, morphdom diffs against the server's class="card" (which is already present in toEl), so the copy loop skips it — and the user's open class is lost on the next diff.
• The same applies to style, aria-*, and any other attribute the server template explicitly emits.

The behavior is correct and intentional (server retains authority over attributes it declares), but the attribute name lvt-preserve-attrs implies broader preservation than what is delivered.

Suggested action: Add a one-liner to user-facing docs/README that spells out the exact guarantee:

lvt-preserve-attrs preserves only attributes the server template does not emit at all. Attributes present in the server template are always overwritten by morphdom.

Also consider adding a test case that explicitly documents this limit (attribute present in server template is NOT preserved, showing the boundary).

Original Comment

lvt-preserve-attrs silently overwrites JS-mutated attributes that the server also controls. The comment notes this already, but it's easy to misuse: if the server template sets class="card" and JS later adds card open, morphdom diffs against toEl's class="card" (which already has the attribute), so the copy loop skips it and open is lost. Since this is a footgun with no runtime signal, the attribute docs should warn that it only protects attributes the server template does NOT emit at all.

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This issue was automatically created by prmonitor from PR review comments.