The html_tag render element supports nested render arrays

The html_tag render element now supports render array nesting, similar to how arrays are nested under container elements.

Before

Previously the html_tag only accepted markup in its #value property, which meant you had to pass a string or pre-render markup.

$rect = [
  '#type' => 'html_tag',
  '#tag' => 'rect',
  '#attributes' => [
    'width' => 50,
    'height' => 50,
    'style' => 'fill:rgb(0,0,255)',
  ],
];
$svg = [
  '#type' => 'html_tag',
  '#tag' => 'svg',
  '#value' => render($rect)
];

After

Now you can nest render arrays under an html_tag, allowing more complex structures.

$svg = [
  '#type' => 'html_tag',
  '#tag' => 'svg',
  [
    '#type' => 'html_tag',
    '#tag' => 'rect',
    '#attributes' => [
      'width' => 50,
      'height' => 50,
      'style' => 'fill:rgb(0,0,255)',
    ],
  ],
];