A headline writer must have more than just a knack; he or she “must employ all the arts of compression and allusion to make immediate sense, to attract the reader, and to tell the news”. That’s an introduction on Sir Harold Evans’s website to News Headlines: An Illustrated Guide, one of a series of books Evans wrote about editing and design in the 1970s when he was editor of the Sunday Times. Writing headlines has always been a skilled job, but the web has introduced another layer of complexity.

Consider this entirely accurate headline published on the web on 11 September: “Channel 4 cancels screening of film questioning Islam’s origins”. As the subheading – known by journalists as a standfirst – and the story made clear, a screening of Islam: The Untold Story, a film broadcast on Channel 4 three weeks ago, was cancelled at the station’s headquarters on the basis of security advice.

Inayat Bunglawala, a contributor to Comment is free who has written on his own blog about the film, emailed the journalist who wrote the story – but not the headline – to say: “The headline … has given rise to many ignorant comments on the Guardian website from readers who appear not to have carefully read your story and have presumed that C4 refused to broadcast the Tom Holland documentary (whereas they have already broadcast it and it is still available to view on 4OD). Please can you try and get a sub to possibly amend it? At present it is being used by some nasty rightwing types to create mischief…”

If that strikes any reader as being oversensitive, it is a request that should perhaps be viewed in the context of a US ambassador having been killed in Libya last week amid protests sparked by another film that appears to have gone out of its way to offend Muslims. Any notion that the Muslim community has successfully suppressed a TV programme examining the origins of Islam feeds tensions already present. And that notion is certainly out there. It surfaced quickly on Twitter, and in comments posted under the crystal-clear story, such as this one: “I think it is time to worry when a TV station in a democratic country, where free speech is allowed, shies away from broadcasting a programme upon religion … A victory for theocratic fascism.”

Of course that comment may be either the result of a wilful misreading of the story, or of not reading beyond the headline. But on the web, the headline is often all people see, for example on a search engine or on Twitter. In a world of increasing aggregation, it is disaggregation that often threatens meaning.

Chris Moran, who heads the Guardian’s efforts at search engine optimisation – whereby words in headlines and elsewhere in web articles are chosen so as to guarantee the widest spread of the story – said: “Even before we worry about optimisation of headlines, our subs need to consider the most profound difference between print and web – the almost endless number of places in which headlines will appear out of context. RSS feeds, Twitter and search results are just a few examples. Therefore we strive to tell the story of a piece clearly in every headline and aim to give readers all the help and information they need to make sure they are clicking on something of interest to them.”

Amelia Hodsdon, who edits the Guardian’s style guide with David Marsh, said: “In print, headlines often work only in conjunction with an image – ‘Phew!’, the day after Team GB won its first gold medals, only made sense with the picture of a triumphant, relieved Bradley Wiggins. On the web, when the headline is often all you get until you follow a link, there is little context beyond the words themselves.

“In the Guardian’s style guide entry for headlines, ‘Take care over ambiguity’ is one of the imperatives. Readers should not be expected to have to reread headlines to understand them – or to necessarily read the standfirst, which is easily divorced from it online. For those who work in journalism, standfirsts are a vital part of the ‘furniture’ that summarises a story, fleshing out the necessarily limited details in the headline, but to many readers the headline is all – and should, on its own, be a fair representation of the story.

“Though in conjunction with the standfirst the meaning of the headline on the story about the Channel 4 film is clear, on its own it is ambiguous. If a headline can be read wrongly, it will be – and in this case, on such a contentious topic, extreme care should have been taken.”