docs: clarify language and direction inheritance in shadow DOM

[Kin230k]

Description

Clarify the existing Attribute inheritance section in the Using shadow DOM guide. Explain parent-based inheritance across a shadow boundary and distinguish a slot's inheritance from that of its assigned light-DOM content.

Motivation

The current sentence implies that every slot inherits language and directionality directly from the shadow host, overlooking intervening shadow-tree ancestors. The replacement adds a short nested-slot example and retains qualifications and reference links for directionality exceptions, following the invitation to propose a focused update.

Additional details

The wording follows the HTML Standard's language rules and parent directionality algorithm, together with the DOM Standard's slot assignment rules.

Validation:

• Changed-file Markdown, Prettier, front-matter, spelling and cross-reference checks passed.
• npm test: 12 passed, 0 failed.
• Changed-page Rari build and Fred SSR passed with no reported page issues.
• Local rendered page checked at 1440px and 390px: both paragraphs and links rendered correctly, without horizontal overflow.
• A Chromium experiment checked :lang(), :dir(), DOM parents and explicit attributes separately for nested slots, fallback content, assigned spans, explicit overrides, and directionality exceptions (bdi and telephone inputs).

This is a one-file prose correction: no runtime, dependency or anchor changes. No cross-browser or assistive-technology testing is claimed.

AI assistance: Codex assisted with research, wording, validation and a separate exact-commit review.

Related issues and pull requests

Fixes #45249

[Josh-Cena]

Thank you, great additions

[github-actions]

Preview URLs (3 pages)

• /en-US/docs/Web/API/Web_components/Using_shadow_DOM
• /en-US/docs/Web/HTML/Reference/Global_attributes/dir
• /en-US/docs/Web/HTML/Reference/Global_attributes/lang