Adventures in the Shouting Style
As the poster's example from John Kirton, The Standard Speaker & Elocutionist (1880) indicates, writers have long been aware of the possibility of using all caps as a way of indicating shouting:
This will be seldom needed throughout an entire piece, but whenever the words imply calling, or commanding, it will be in keeping with the words to employ it. As examples note the following selections [from Tennyson, Scott, Halleck, Byron, and R.G. Conrad] marked in CAPITAL letters as the appropriate place for shouting emphasis.
Kirton's discussion of "Shouting Style" is strikingly similar to that of S. S. Hamill, The Science of Elocution (1872), who uses italics rather than all caps to indicate the words to be shouted:
The Shouting Style is chiefly used in the utterance of those words and phrases which are employed in calling and commanding. But few selections will require the Shouting Style throughout.
In the extracts given [from Tennyson, Scott, Halleck, Boker, and Knowles], with the exception of Tell's Address to the Alps, only those words printed in italics require the Shouting Style.
So if you're wondering, "When did italic formatting come to indicate shouting?" the answer is, no later than 1872. It's a pity that neither Kirton (writing in England) nor Hamill (writing in Illinois) acknowledges the other author, because one of them owes the other a considerable debt. Preliminarily, at least, it appears that Kirton is the debtor.
Acknowledging that all caps can mean shouting is a far cry from interpreting all caps as indicating shouting in every instance. For example, the vast majority of cartoons in daily newspapers (Doonesbury, Pearls Before Swine, Candorville, Garfield, Mutts, etc.) present all of their dialogue in all caps—and yet few if any readers interpret the dialogue as nonstop shouting. That's because, by cartoonist convention (or cartoonist folk wisdom), all caps lettering is simply more readable than uppercase/lowercase lettering. When cartoonists want to indicate shouting, they use boldface all caps, or enlarge the size of the words relative to the surrounding text, or both.
All caps for visibility or emphasis
For centuries, authors have used all capitals for visibility or for emphasis. John Flavel, The Fountain of Life Opened, Or, A Display of Christ in His Essential and Mediatorial Glory (1671) devotes a chapter to "The Title Affixed to the Cross of Christ," which begins as follows:
"And a superscription also was written over him in letters of Greek, and Latin, and Hebrew, This is the King of the Jews." Luke, 23 : 38.
Before I pass on to the manner of Christ's death I shall consider the title affixed to the cross, in which the wisdom of Providence was strikingly displayed. It was the manner of the Romans, that the equity of their proceedings might the more clearly appear to the people, when they crucified any man, to publish the cause of his death on a tablet written in capital letters, and placed over the head of the victim. And that there might be at least a show of justice in Christ's death, he also has his title or superscription.
And "Debates in the Senate of Lilliput," published in The Gentleman's Magazine (February 1742) includes this example:
In a speech which he made against a Motion for keeping up 16,000 standing Forces, he said, I know what is laid down in his Majesty's Speech. But we are to consider that speech, as the Composition and Advice of his Ministry, and are therefore at Liberty to debate every Proposition in it ; ESPECIALLY THOSE WHICH SEEM RATHER CALCULATED FOR THE MERIDIAN OF ALLEMANNU THAN OF GREAT LILLIPUT.
'Tis the only Infelicity of his Majesty's Reign, THAT HE IS UNACQUAINTED WITH OUR LANGUAGE AND CONSTITUTION ; and 'tis therefore more incumbent on his Lilliputian Ministers to inform him that our Government does not stand on the same Foundation with his Allemannuan Dominions, which by Reason of their Situation an the Nature of their Constitution are obliged to keep up Armies in time of Peace. ... These Expressions gave Offence to several Members, and one having taken these Words marked in Capitals down in Writing, urged, that they were a scandalous Invective against his Majesty's Person and Government, of which the House ought to shew the highest Resentment, and therefore Moved, that the Member who spoke those offensive Words should be sent to the Tower of Mildendo.
The Roman law specifying writing the cause of death for crucified criminals in all caps was evidently prompted by a desire for the cause to be easy to read from a distance (the same rationale that causes states to use all caps for letters on license plates). The purpose of using all caps in the item from The Gentleman's Magazine was evidently to emphasize the offensive passages of a parliamentary speech, but there is no implication that the person whose words are quoted shouted those particular phrases at a higher decibel level than he used for the rest of his remarks.
The drawbacks of using all caps for emphasis
Dougan Laird, Writing for Results: Principles and Practice (1978) [combined snippets] criticizes the use of all caps for the length of a sentence not because he interprets it as shouting, but because he says it flattens out the text by making internal emphasis harder to represent:
The reason dashes should be a rare punctuation mark is just that when too many things are emphatic, nothing is emphatic. Why not look at the next sentence for proof? Sometimes we use ALL CAPITAL letters to emphasize; IF WE CAPITALIZE ALL THE LETTERS OF EVERY WORD, WHICH WORDS ARE REALLY EMPHATIC? They all look just the same, don't they?
Writing 63 years earlier, Epes Sargent, Picture Theatre Advertising (1915) makes a similar point:
If you want to gain additional emphasis, use italic or full face in preference to all capital letters. This applies also to too much gothic type, most of which is cut with a single case, the upper. ... The objection to an all-capitals line is not the fact that they are capital letters, but that all capital letters are of the same height and there is no variety to the eye. There are no ascenders to serve as mileposts. There is nothing to hang onto the line by. ... All capital letters do not give emphasis to a line. They serve merely to hide the facts you are trying to tell.
All caps as an option for indicating shouting
In the days before online communication, some writers certainly understood that they could use all caps to signify shouting. For example, Don Gillis, The Art of Media Instruction (1973) [combined snippets] offers this discussion:
In written language, capital letters assume an implied importance (such as capital "H" when used with reference to God). There are strict rules governing our use of capitals, and we adhere to them in order to be clearly understood. Mechanically reproduced letters (such as print or typescript) limit, to some extent, the meaningfulness of letter-making since it is depersonalized when compared to the manuscript or handwritten copy When typing we may shout by SHOUTING with capitals, subject to the limitations of a machine. We can shout with the letters themselves only to the extent of the mechanical latitude. In handwriting we can shout all over the paper if we like, although most of us conform to a relationship of mechanically produce lettering.
But Gillis isn't claiming that every instance of all caps should be interpreted as shouting; he is simply saying that writers may use them in this way.
When ad copy invites readers to "get your FREE sample of this incredible product," the point is to make "FREE" look emphatic and eye-catching, not to be interpreted as shouting. And in fact, when pitchmen in radio and TV ads actually read such copy, they almost never shout the emphasized word (relative to the other words in the script); they stress the word and linger on it, but they don't double the volume of their voice.
All caps as a convention to indicate shouting
The informal convention or understanding that words typed in all caps automatically and universally signify shouting appears to be an innovation of the computer age. That the convention was widely shared in computing circles by 1996 is evident from this brief discussion in Eric Raymond, The New Hacker's Dictionary, third edition (1996):
Hackers have also developed a number of punctuation and emphasis conventions adapted to single-font all-ASCII communications links, and these are occasionally carried over into written documents even when normal means of font changes, underlining, and the like are available.
One of these is that TEXT IN ALL CAPS IS INTERPRETED AS 'LOUD', and this becomes such an ingrained synesthetic reflex that a person who goes to caps-lock while in talk mode may be asked to "stop shouting, please, you're hurting my ears!".
Unfortunately, I don't have access to earlier versions of this dictionary, so I can't say whether Raymond includes the same observation in earlier editions. In any case, the understanding of all-caps as shouting was so thoroughly established by 1996 that Raymond cited it as an example of a universal (among hackers) convention.
A slightly earlier computer-related discussion of all caps appears in Philip Baczewski, The Internet Unleashed (1994) [combined snippets] :
Usenet even allows people to shout at each other! Most posts are properly typed using upper- and lowercase letters—as one would expect (although some users seem to ignore the use of the Shift key entirely). When a user wants to emphasize something, it is usually written in capital letters. This is the Usenet equivalent of shouting.
It is considered bad practice to shout excessively. Shouting works much better when you emphasize the important word or phrase, but running uppercase letters for more than a sentence is not advisable. It may result in you getting yelled at!
In the late 1990s, I worked at a computer magazine. One of our tech editors had a habit of marking up manuscripts with comments in all caps—until some younger editors complained to him that he was shouting at them. He seemed genuinely surprised and distressed that they interpreted his use of all caps in that way, which suggests to me that, as late as 1998, not everyone working with computers had gotten the memo that ALL CAPS = SHOUTING. Today, that equivalence is common knowledge even among latter-day AOLers.
The occasional use of all caps as an explicit way to indicate words to be given "shouting emphasis" clearly goes back to at least 1880. But the idea that an instance of all caps necessarily signifies shouting was, I think, something new under the sun when computer enthusiasts of the Usenet era adopted it as a convention. I don't know when this convention began to be widespread, but it was undoubtedly well established by 1994 and may have begun many years before that.