In a document with HTML markup like this:

<p>Some <em>emphasized</em> text</p>

The <p> generates a block box, with several inline boxes inside
it. The box for “emphasized” is an inline box generated by an inline
element (<em>), but the other boxes (“Some” and “text”) are inline boxes generated by a block-level element (<p>). The latter are called anonymous inline boxes, because they don’t have an associated inline-level element.

Such anonymous inline boxes inherit inheritable properties from
their block parent box. Non-inherited properties have their initial
value. In the example, the color of the anonymous inline boxes is
inherited from the P, but the background is transparent.

Whitespace content that would subsequently be collapsed away according to the ‘white-space’ property does not generate any anonymous inline boxes.

If it is clear from the context which type of anonymous box is
meant, both anonymous inline boxes and anonymous block boxes are
simply called anonymous boxes in this specification.

There are more types of anonymous boxes that arise when formatting
tables.

Cascading Style Sheets, level 2 revision 1
CSS 2.1 Specification
W3C Working Draft 13 June 2005
Visual formatting model
http://www.w3.org/TR/CSS21/visuren.html#inline-level