One of the features that characterizes usage guides of the 20th and 21st centuries is an admonition to write in plain style. The features of plain style consist of: shorter sentences rather than longer (think Hemingway, not Faulkner); concrete words rather than abstract; native words rather than borrowed ones (there's often an assumption that native words are more concrete, borrowed ones more abstract). Plain style is contrasted with high style (Aristotle) or florid style (euphuism or inkhorrnism, for example).
The King's English (1906). Campbell (1776) called for language that is reputable, current, and national. But more recent commentators specify exactly what they're looking for. Here are some examples of plain style recommendation, with the most important first, the least important, last:
from Henry and Francis Fowler, The King's English (1906), p. 1.
But, and this is an important but, it's more important to use language that makes sense than to follow any of the rules. The Fowlers give these examples: don't use englishing when you mean translation. True, englishing is a native word, while translation is borrowed. But it's strange, unusual, far-fetched. It will make readers stop and wonder, interrupting the flow of the sentence. Don't use foreword instead of introduction. Some writers, they note, are throwing out borrowed words like introduction. The Fowlers disapprove. They recommend leaving such practices to the Germans, who have been more successful in nativizing their language. English, they claim, doesn't need this. Interestingly, while a number of 19th-century English writers like William Barnes preferred ditching foreign words, most English writers ignored them. But foreword is one of the few nativistic coinages that English writers did pick up. (Another was handbook, instead of the borrowed manual.) Apparently the Fowlers didn't see this coming, or they just didn't like foreword, which today is a perfectly normal English word.
Politics and the English Language (April 1946, Horizon Magazine). In his often-reprinted essay, George Orwell complains that political writers propagandize, and that their style itself is a form of propaganda. He recommends five rules to correct such tendencies, with a sixth designed to eliminate excess:
Orwell does not cite his dependence on the Fowler brothers for these rules. There are now two annual Orwell Prizes for political writing, one for a book and one for journalism.
Strunk and White: The Elements of Style (1919; 1959 - present). William Strunk taught English 8 at Cornell in the early 1900s. The text for this composition course was a little book which he published himself, The Elements of Style. E. B. White took the class in 1919. In 1957, a friend sent White a copy of Strunk's textbook, stolen from the Cornell library. White then wrote about the book in his New Yorker column. A publisher convinced White to edit Strunk's book and bring it up to date. The Elements of Style, often called simply Strunk and White, appeared in 1959 and has gone through four editions, selling more than 10 million copies. Strunk and White advocate for plain style as well. Here are some of their rules, from the fourth edition:
All except the third rule appear in Strunk's original version of the text. A preference for the active voice begins to appear in usage guides in the early 20th century. This seems to go along with the tendency to see shorter sentences as features of modern prose style (the active version of a sentence is usually shorter than the passive, unless the agent is omitted from the passive).
These kinds of rules, especially the preference for the active voice and the concrete word, tend to be repeated endlessly. Admonitions like "keep related words together" lead to rules like, "don't end a sentence with a preposition" (even though it is stated negatively!), despite the fact that sentence-final prepositions are often necessary in English.
In contrast, sensible reminders that usage rules are really suggestions rather than commandments are not stressed. All the usage critics acknowledge what Orwell put so bluntly: "Break any of these rules sooner than say anything outright barbarous." Orwell means, if following a rule leads to a cumbersome, awkward, or difficult sentence, then don't follow the rule. Use your ear. All writers do that. They don't consult rule books. Beginning writers must learn to use their ear as well -- depending on rule books, with their formulas, cannot produce the plain or modern style, and they certainly can't produce good writing.
Even so, writers cling to books like The Elements of Style, and they swear by Orwell's "Politics and the English language" -- often without really reading it. Ask a fan of Strunk & White or Orwell what they think of the work knowing that both were based on stolen ideas (and in White's case, on a stolen book), and see what they say.