British english or American english, HTML

Does this mean HTML is written in "American English"?

Yes.

 align = left|center|right [CI]

However, using the align property is deprecated - consider using CSS's text-align.

Don't think of the values as English words, think of them as enumerated values. The HTML standard explicitly defines certain strings that are valid.

If you don't know what the acceptable values are, you need to look them up. If you want to guess at the accepted values then you should try the American English spellings.