Gmail supports several inline underline styles through both toolbar options and keyboard shortcuts. Here's a comprehensive overview of what's available:

Basic Underline Options

• Standard underline (Ctrl+U or ⌘+U on Mac): Creates a single line under text
• Double underline: Available through the formatting toolbar
• Strikethrough (technically a style of underline): Shows text with a line through it

Keyboard Shortcuts

Advanced Styling

1. Color Customization
   • Text color can be changed independently from underline color
   • Available through the text color picker in the toolbar
2. Font Combination
   • Can combine underlines with other font styles
   • Works with bold, italic, and strikethrough simultaneously

Implementation Methods

1. Toolbar Method
   • Select text
   • Click the underline button in the formatting toolbar
   • Choose desired style from dropdown
2. HTML Method
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   <!-- Standard underline -->
   <span style="text-decoration: underline;">Underlined text</span>
   
   <!-- Double underline -->
   <span style="text-decoration: underline double;">Doubly underlined text</span>

Limitations

• Custom underline colors aren't directly supported
• Wavy or dotted underlines aren't available
• Some advanced CSS underline properties won't render in emails