All caps is a very tempting way to make a headline or other block of text stand out from the rest of the text. But writing headlines in all capital letters ends up making the text more difficult to read than if it were written in standard title case.
Compare Two Sentences
In the image in this article I’ve written two sentences with the last word removed. What do they both say? The first is in all lower case, and the second has the missing word in all caps.
If you guessed “My child is a boy.” for the first sentence, you would be correct. There are other words that could fit in that shape, but “boy” makes the most sense. But what about the second sentence? For that sentence, I wrote the last in all capital letters. It might read “My cat is a GIRL.” or it could say “My cat is a MANX.” Or something else entirely.
We Read Words Not Letters
When we read blocks of text, most people read the shapes of words rather than letter by letter. As you can see from my image, words in lower case have distinct shapes. The letters drop below and rise above the lines and create distinct shapes. All caps creates rectangles. Every word has the same shape.
When you write something in all capital letters your reader has to read each letter individually to form the words. This means that the headlines take longer to read.
All Caps Means Shouting
Most people see all caps and “hear” the words as shouting. If all you’re trying to do is emphasize the text, you should use CSS to decorate the headline or add the emphasis. Some people don’t like being shouted at, and text emphasized with CSS appears more friendly.