html: Usage of optgroup tag, grouped, read-only and displayed in tree style

Usage of optgroup tag in html, Group, read-only and displayed in tree style

in html In addition to the option tag in the select tag, there is another tag called optgroup. What can it do for us?

We have already understood the meaning of connecting points from the name: it is an option group, just like checkboxgroup and radiogurop, it is used for grouping ;    

Look at the following piece of code, open it in a browser and you can see a drop-down menu. But when you choose java or .Net, you can't choose.​​​

And the style is different from that of the option tag, and the font is bold. And the drop-down menu is displayed in a tree-like style.

  
  
      
          
          
          
          
    
  
      
          
              
                jsp Servlet  
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