39.12.4 Displaying Faces

When Emacs displays a given piece of text, the visual appearance of the text may be determined by faces drawn from different sources. If these various sources together specify more than one face for a particular character, Emacs merges the attributes of the various faces. Here is the order in which Emacs merges the faces, from highest to lowest priority:

• If the text consists of a special glyph, the glyph can specify a particular face. See Glyphs.
• If the text lies within an active region, Emacs highlights it using the region face. See Standard Faces.
• If the text lies within an overlay with a non-nil face property, Emacs applies the face(s) specified by that property. If the overlay has a mouse-face property and the mouse is near enough to the overlay, Emacs applies the face or face attributes specified by the mouse-face property instead. See Overlay Properties.

  When multiple overlays cover one character, an overlay with higher priority overrides those with lower priority. See Overlays.

• If the text contains a face or mouse-face property, Emacs applies the specified faces and face attributes. See Special Properties. (This is how Font Lock mode faces are applied. See Font Lock Mode.)
• If the text lies within the mode line of the selected window, Emacs applies the mode-line face. For the mode line of a non-selected window, Emacs applies the mode-line-inactive face. For a header line, Emacs applies the header-line face.
• If the text comes from an overlay string via before-string or after-string properties (see Overlay Properties), or from a display string (see Other Display Specs), and the string doesn't contain a face or mouse-face property, but the buffer text affected by the overlay/display property does define a face, Emacs applies the face attributes of the “underlying” buffer text. Note that this is so even if the overlay or display string is displayed in the display margins (see Display Margins).
• If any given attribute has not been specified during the preceding steps, Emacs applies the attribute of the default face.

At each stage, if a face has a valid :inherit attribute, Emacs treats any attribute with an unspecified value as having the corresponding value drawn from the parent face(s). see Face Attributes. Note that the parent face(s) may also leave the attribute unspecified; in that case, the attribute remains unspecified at the next level of face merging.