Document.firstElementChild
 The Document.firstElementChild read-only property returns the document's first child Element, or null if there are no child elements. 
For HTML documents, this is usually the only child, the root <html> element.
See Element.firstElementChild for the first child element of specific elements within a document.
Syntax
// Getter element = document.firstElementChild; // No setter; read-only property
Example
document.firstElementChild; // returns the root <html> element, the only child of the document
Specifications
Browser compatibility
| Desktop | Mobile | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | WebView Android | Chrome Android | Firefox for Android | Opera Android | Safari on IOS | Samsung Internet | |
firstElementChild | 
29  | 
17  | 
25  | 
No  | 
16  | 
9  | 
≤37  | 
29  | 
25  | 
16  | 
9  | 
2.0  | 
See also
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    https://developer.mozilla.org/en-US/docs/Web/API/Document/firstElementChild