title attribute
The title attribute may be coded with the expanded meaning of the acronym or abbreviation in the content of the <abbr> tag. However, browsers tend to ignore the title attribute of the <abbr> tag. To have the browser pop up tool tips with the meaning of acronyms and abbreviations wrap the <abbr> tags inside <a> tags with title attributes instead, as described by Acronyms .net.
Unlike the common attribute of the same name, the title attribute of the <dfn> tag does not inherit its value from an ancestor tag.
common attributes
In addition to the attributes above, which are specific to the <abbr> tag, any of the common HTML attributes can also be used.
When an acronym or abbreviation is associated with a definition, it could be either the abbreviated form or its expanded meaning that is being defined. The term being defined is identified by the value of the title attribute of the <dfn> tag.
When an abbreviation is a short form of the actual term being defined, a parent <dfn> tag will also assume the value of the <abbr>'s title attribute, which indicates the term that is being defined is the fully expanded meaning:
<dfn><abbr title="Internet Corporation for Assigned Names and Numbers">ICANN</abbr></dfn>is the international organization which helps ensure that Internet domain names are assigned in an orderly manner.
When an acronym's meaning is being clarified by expanding it, then it is the acronym itself that is being defined and the title attribute of the parent <dfn> tag also needs to be coded to indicate it is not the expanded meaning in the <abbr>'s title but the acronym or abbreviation itself that is being defined:
When we use the acronym <dfn title="RSS"><abbr title="Really Simple Syndication">RSS</abbr></dfn>it is the abbreviation for <b>Really Simple Syndication</b>, not RDF Site Summary.
<abbr> should be used instead of <acronym> since the acronym tag has been deprecated.