Fix html_attr dropping style declarations whose value is zero

InlineStyle::getValue() skipped a declaration when empty($value) was true,
which also matches 0, 0.0 and '0'. Those are ordinary CSS values (opacity: 0,
z-index: 0, margin: 0, flex-grow: 0), so they were silently dropped, and a
style map containing only such declarations omitted the attribute entirely.

The sibling SeparatedTokenList::getValue() already uses an explicit
null/false test, so class token lists keep a 0 while style declarations did
not. The numeric-key branch of InlineStyle itself never consulted empty(),
so {style: ['opacity: 0']} printed while {style: {opacity: 0}} did not.

CHANGELOG

@@ -1,5 +1,6 @@
 # 3.29.0 (2026-XX-XX)
 
+ * Fix `html_attr` dropping `style` declarations whose value is `0`, `0.0` or `'0'`
  * Fix the `default` filter fallback emitting an undefined variable warning when it uses the null-safe operator
  * Fix the `matches` operator silently treating PCRE execution errors as non-matches
  * Add the `HtmlExtension::htmlAttrValue()` method to resolve a single HTML attribute value the way the `html_attr` function renders it

extra/html-extra/HtmlAttr/InlineStyle.php

@@ -55,7 +55,9 @@ final class InlineStyle implements MergeableInterface, AttributeValueInterface
     {
         $style = '';
         foreach ($this->value as $name => $value) {
-            if (empty($value) || true === $value) {
+            // `0`, `0.0` and `'0'` are valid CSS values, so only the values that carry
+            // no declaration at all are skipped here
+            if (null === $value || false === $value || true === $value || '' === $value || [] === $value) {
                 continue;
             }
             if (is_numeric($name)) {

extra/html-extra/Tests/HtmlAttrTest.php

@@ -157,6 +157,27 @@ class HtmlAttrTest extends TestCase
             ],
         ];
 
+        yield 'zero style declaration values are printed' => [
+            'style="opacity: 0; z-index: 0; margin: 0;"',
+            [
+                ['style' => ['opacity' => 0, 'z-index' => '0', 'margin' => 0.0]],
+            ],
+        ];
+
+        yield 'null, false and empty string style declaration values are omitted' => [
+            'style="color: red;"',
+            [
+                ['style' => ['a' => null, 'b' => false, 'c' => '', 'd' => true, 'color' => 'red']],
+            ],
+        ];
+
+        yield 'style attribute is omitted when every declaration is omitted' => [
+            '',
+            [
+                ['style' => ['a' => null, 'b' => false, 'c' => '']],
+            ],
+        ];
+
         yield 'merging style attributes overrides by key' => [
             'style="color: blue; font-size: 14px;"',
             [