HTML: Optgroup Tag - scmGalaxy
The optgroup in HTML select forms in Dreamweaver - Web Teacher
How to specify a label for an optgroup tag in HTML5 ? - GeeksforGeeks
File:HTML-select-optgroup-example-gl.png - Wikimedia Commons
Html.optgroup lacks "label" attribute · Issue #136 · elm/html · GitHub
Quick Tip: Don't Forget the “optgroup” Element
Styling single selection menu control option groups (optgroup elements) with CSS | 456 Berea Street
HTML | <optgroup> label Attribute - GeeksforGeeks
HTML <optgroup> disabled Attribute - GeeksforGeeks
optgroupタグの意味と使い方 | HTML | できるネット
The optgroup in HTML select forms in Dreamweaver - Web Teacher
javascript - How to Dynamically Create <option> elements within a parent < optgroup> - Stack Overflow
Quick Tip: Don't Forget the “optgroup” Element
Quick Tip: Don't Forget the “optgroup” Element
Jquery bootstrap multiselect optgroup selection - Stack Overflow
Styling single selection menu control option groups (optgroup elements) with CSS | 456 Berea Street
HTML <optgroup> Tag - GeeksforGeeks
javascript - Bootstrap optgroup selection - Stack Overflow
Grouping Form Elements in HTML: Combobox UI Control for Webpage.
