As I mentioned in What Edison can teach us about SEO, the first rule of writing is following through on what you promise in the title. If you fail to do that, you're guilty of bait-and-switch, or what is commonly known as clickbait.
The promise the reader is not just about the topic covered but the approach and extent promised in the title. Consequently, if the title promises to explain a concept in 300 words, how long should it be? Just 300 word, not 500, and certainly not over 1800 words. Yet that's just what HubSpot did with its article entitled What Is an API? The Answer in 300 Words or Less.
That's Application Programming Interface (API) which connects many functions across many forms of business.
In truth it's a very thorough article that's been updated more than once to grown more comprehensives since it was first published in 2013. However, the promise of the title is not "Everything you need to know about an API" but a super-short overview that can be read in under a minute. For the current length, the article is described as a seven minute read.
What HubSpot should have done in this case is retain whatever short form the article may originally have held and then offered a link to the longer form with a CTA introduced along the lines of, "That's APIs in a nutshell. If you want to understand more about how they work for marketing, take a deep dive into the subject here."
