A "grammar" is a set of rules that speakers implicitly agree to use so they can understand each other. Linguists distinguish between descriptive and prescriptive grammars. Most languages have a written form and a spoken form.
A grammar does not have to describe sound or text: visual language also has a grammar and even a phonology! These hand-icons are used to establish hand-shape for an ASL word, "snow-boarding."
A UCLA course hand-out surveys if you use "speaking too slowly" or "speaking too slow" and gives more examples that contrast the two grammars you keep in your head for English.
Computer-based human communication welds elements from both grammars to provide casual sounding, but informative text. Perhaps, a third grammatical category is in order for telegraphic text. Language Educator has an Oct 2009 article in English on how to form abbreviations for texting in Spanish.
