From the most recent edition of the Mornington Crescent Handbook (reproduced with permission):

Section T. Playing Online
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3.1a Howells Bold Convention – highlighting the station being played with the Bold/Strong tags, eg. Tottenham Hale.
3.1b Howells Starred Convention – highlighting the station being played with asterisk, eg. *Harlesden*.
3.1c Howells Capitals Convention – highlighting the station being played by typing it in capitals, eg. EAST ACTON.
3.2 The Howells Convention to be played on bulletin boards or forums should be mentioned as early as possible, or should be assumed from the first move.
3.3 Moves that are not highlighted in the Howells Convention can be deemed illegal, although players discretion is advised.