XML-TEI tagging is semantic tagging, not formatting tagging. In other words, the tags indicate the meaning or function of an element within a text, not the expected final appearance or presentation, such as italics, bold, small caps, sizes, spacing, margins, typefaces, etc.

An XML-TEI document contains the text we are editing without any formatting, it is only semantic. The format of the final presentation will depend on the conversion platform you want to use: web page, PDF book, audio book, ePub, etc. Each of these media will apply their own styles and aspects, and our XML-TEI has to be compatible with any type of final presentation.

For example, in traditional publications it is common to italicise titles of works, or words in foreign languages, or terms that want to be highlighted in some way. That is, italics means different things depending on the context, but in itself means nothing.

This ambiguity is resolved by semantic tagging, which, while indicating that a text is a title does not say that it is italicised, but that it is a title with <title>, if it is a term in another language it will be a <foreign>, and if it is a highlighted text it is labelled <emph>. The format in which it is displayed will depend on the style of the final publication, and may be italics or bold or whatever you wish. But the important thing is not that they are italicised, but their actual meaning.

However, there are contexts in which it is necessary to indicate the format, when it is a text highlighted within another, a title within another, or when there is no semantic tag that reflects the meaning of the element.