The codepoints below U+0020 are not graphic characters, they're non-printing controls.

Per CSS Text,

Control characters (Unicode category Cc)—other than tabs (U+0009), line feeds (U+000A), carriage returns (U+000D) and sequences that form a segment break—must be rendered as a visible glyph which the UA must synthesize if the glyphs found in the font are not visible, and must be otherwise treated as any other character of the Other Symbols (So) general category and Common script. The UA may use a glyph provided by a font specifically for the control character, substitute the glyphs provided for the corresponding symbol in the Control Pictures block, generate a visual representation of its code point value, or use some other method to provide an appropriate visible glyph.

(Our behavior is controlled by layout.css.control-characters.visible, which we'd like to make true everywhere but other browsers have been slow to align properly.)