1. Abbreviations & Numbers Rules
Write Source
Abbreviations & Numbers Rules

2. Numbers 658. 1 Numerals or Words: 658.2 Numerals Only 658.3 Words Only
numbers from one to nine are usually written as words
numbers 10 and over are usually written as numerals.
numbers being compared or contrasted should be kept in the same style (ex. 8 to 11 years or eight to eleven years)
Use a combination of numerals and words for very large numbers (1.5 million)
If numbers are not used frequently in a writing, spell out those numbers which can be written in two words or less.
658.2 Numerals Only
decimals, percentages, chapters, pages, addresses, phone numbers, identification numbers, and statistics.
658.3 Words Only
to express numbers that begin a sentence.
658.4 Time and Money
Time: use numerals if expressed with an abbreviation; spell out the numeral is expressed in words (4:00 A.M. or four o’clock)
If an amount of money is spelled out, so is the currency; use a numeral if a symbol is used ( twenty dollars or $20)

3. Abbreviations 660.1 Formal and Informal Abbreviations
Abbreviation: shortened form of a word or phrase.
Some abbreviations are acceptable in formal and informal (Mr. Mrs. Jr. Ms. Dr. a.m. (A.M.) p.m. (P.M.)
Not abbreviated: states, countries, months, days, units of measure. (reference note on 660)
Correspondence Abbreviations
United States, Canadian Provinces, Addresses, Other

4. Acronyms and Initialisms (WS 662)
Acronym - (a type of abbreviation) A word formed from the initial parts (letters OR syllables OR arbitrary parts) of a name. Examples: NATO (North Atlantic Treaty Organisation)
Initialism - (a type of abbreviation) A group of initial letters used as an abbreviation for a name or expression, each letter being pronounced separately. For example, "BBC" (British Broadcasting Corporation), or "PBS" (Public Broadcasting System).

5. Acronym or Initialism?
So, here's your first test: "HTML": abbreviation, acronym, or initialism?

6. Acronym or Initialism?
Well, it is an abbreviation, because it's a shortened form of "Hyper-Text Markup Language".

7. Acronym or Initialism?
But it's not an acronym, because it's not a word! (You can tell, because you don't say "heh-teh-mehl"; you say "aych-tee-ehm-ehl".)

8. Acronym or Initialism?
And it is, as you may have guessed, an intialism, for the same reason it's not an acronym (you pronounce each letter separately).