Newleaf supports various ‘extra’ formatting which is applied to the existing font definition. This currently includes bold, italic, underline and strikethrough. To underline an entire paragraph (you might want to include this in a subhead style):

nlEmphasis 'underline'
nlPlace 'Subheading'
nlSet 'emph' ''
'Title' nlDefineStyle ('fon' 'tii,24')('al' 'ce')

    ('sp' 18 12)('pagebr' 1)
'Subhead' nlDefineStyle ('fon' 'heb,14')('space' 6 3)

    ('emph' 'underline')
'subhead' nlPlace 'Section 4.3'

If you prefer, you can pass the emphasis as a numeric code, being the sum of:

• 1=bold
• 2=italic
• 4=underline
• 8=strikethrough

The underlined option also works as an HTML tag in the same way as bold and italic:

nlFlow 'Here is <u>underlined</u> text'.

Strikethrough is not well supported by HTML, and will not be included in the set of supported tags.