Web IDL spec says:

For all of these constructs [named definitions, attributes, typedefs, dictionary members, constants, and operations], the identifier is the value of the identifier token with any leading U+005F LOW LINE ("_") character (underscore) removed.

However, webidl2.js no longer seems to remove some underscores, from identifiers that did not need them to start with:

> require('webidl2').parse('interface _Int {};')
[ { type: 'interface',
    name: '_Int',
    partial: null,
    members: [],
    trivia:
     { base: '', name: ' ', open: ' ', close: '', termination: '' },
    inheritance: null,
    extAttrs: null },
  { type: 'eof', trivia: '' } ]