1. The WWP currently uses <abbr> with its expan= value to encode abbreviations whose unexpanded meaning is not clear to the modern reader, or whose expanded value is not obvious. We also use <abbr> to encode renditionally distinct abbreviations, omitting the expan= attribute if the above conditions do not apply. A sample list of abbreviations which do not need expan= (and hence should not be encoded at all if not renditionally distinct) follows. Note that this is a list of examples, not a comprehensive list of all instances:
A.D., Ave., B.C., Bk. (for book) c.f., Ch. or Chap or Cap. (for chapter), Co., d. (for British pence), d. (for ditto), do. (for ditto), Dr., ed., e.g., esp., Esq., etc., &c., et al., ff., H.R.H, ibid., i.e., Inc., l. (for line), l. (for British pounds), lb., ltd., Mr., Mrs., N.B., no., oz., p. or Pag. or pp. or pg. (for page), p.s., Rev. (for Reverend), s. (for British shillings), St. (for either street or saint), U.S.A., viz., vol.
Also not expanded are abbreviations for academic degrees, points of the compass, days of the week, months, and the names of speakers in drama.
2. The period following an abbreviation is included within the <abbr> element. The exception to this rule is that where the abbreviated word falls at the end of a sentence, the period is treated as a sentence-ending period, not as an abbreviatory period, and is placed outside the <abbr> element.
3. The <abbr> element is applied to the smallest meaningful unit which is abbreviated, whatever that may be in the given case:
--a single brevigraph such as a letter with a macron or superscript: <abbr expan="em">ē</>
--a word-length abbreviation: <abbr expan="Obedient">Ob’t.</>
--a phrase: <abbr expan="United Kingdom">U.K.</>
4. The <abbr> element is also used to expand abbreviated references to texts such as the Bible or the classical authors.