[css-fonts-5] Define behavior for <meta text-scale> from #12380

css-fonts-5/Overview.bs

@@ -59,7 +59,63 @@ rules defined in CSS Fonts Level 4.
 This specification is currently a delta to the CSS Fonts Level 4 specification.
 Do not assume that if something is not here, it has been dropped.
 
+<h2 id="text-scale-meta">
+Text-Scale <code class=html>&lt;meta&gt;</code> element</h2>
 
+A document with a <code>&lt;meta></code> tag whose <code>name</code> attribute
+is a <a>ASCII case-insensitive</a> match for
+<dfn lt=text-scale><code>"text-scale"</code></dfn> is recognized as setting the
+initial font size of the document. The value of the <code>content</code>
+attribute must be an <a>ASCII case-insensitive</a> match for one of the
+recognized keywords.
+
+Documents without this <code>&lt;meta></code> tag will have an assumed default
+value of <code>legacy</code>.
+
+<h3 id="text-scale-meta-keywords">
+Keywords</h3>
+
+The recognized keywords in the [=text-scale=]
+<code class=html>&lt;meta&gt;</code> element are:
+
+<ul>
+	<li><code class="index" lt="legacy!!text-scale-meta">legacy</code></li>
+	<li><code class="index" lt="scale!!text-scale-meta">scale</code></li>
+</ul>
+
+<h3 id="legacy-keyword">The 'legacy' keyword</h3>
+
+The <dfn for="text-scale" export><code>legacy</code></dfn> property is
+recognized in the [=text-scale=] content attribute value.
+
+When the value of the [=text-scale=] content attribute is
+<a for="text-scale">legacy</a> the user agent should set the initial font size
+to 16px multiplied by the font scale factor the user has chosen from any settings <i>provided by the user agent</i>. The ''preferred-text-scale'' value must be 1 on desktop platforms. On mobile:
+<ul>
+	<li>if the operating system provides a text scale setting AND the UA hasn't already applied that factor to the initial font size, ''env()/preferred-text-scale'' returns the mulitplier that the user has chosen in the operating system's text scale setting.
+
+	Note: As of this writing, all combinations of Android, iOS, Gecko, WebKit, Blink satisfy this first condition.
+
+	</li>
+	<li>Otherwise ''env()/preferred-text-scale'' returns 1.</li>
+</ul>
+
+<h3 id="scale-keyword">The 'scale' keyword</h3>
+
+The <dfn for="text-scale" export><code>scale</code></dfn> property is
+recognized in the [=text-scale=] content attribute value.
+
+When the value of the [=text-scale=] content attribute is
+<a for=text-scale>scale</a> the user agent may determine the initial font size
+based on a combination of the operating system's text scale setting and the user agent's text scale setting. The
+''env()/preferred-text-scale'' value must be a number that, when multiplied by
+16px, provides a <<length>> that matches that of the initial font size.
+
+Further, when the value of the [=text-scale=] content attribute is
+<a for=text-scale>scale</a>, the user agent should skip all font-sizing interventions it would otherwise perform in an attempt to automatically honor the user's preferences. E.g. text autosizing on mobile (See [[css-size-adjust#intro]]) and full-application zoom (<a href="https://github.com/w3c/csswg-drafts/blob/main/css-env-1/explainers/env-preferred-text-scale.md#windows-11">popular browsers do this on Windows)</a>.
+
+Note: It is expected that authors will use
+''&lt;meta name="text-scale" content="scale"&gt;'' in stylesheets so that the initial font size will reflect a combination of the user's font preferences, whether those are specified at the OS level or the UA level. The author will then be able to use ''rem'' throughout the page to honor the user's font preferences.
 
 <h3 id="values">
 Value Definitions</h3>