Is it a good idea to highlight the important words in a text on a web page? What impact does it have on the reader? Does it help to understand the text or does it disturb?
Yes. Jakob Nielsen did a study many years ago and found that users like reading text that's easily scannable.
Scanning can save users time. During the study, 15 participants always approached unfamiliar Web text by trying to scan it before reading it. Only 3 participants started reading text word by word, from the top of the page to the bottom, without scanning. Elements that enhance scanning include headings, large type, bold text, highlighted text, bulleted lists, graphics, captions, topic sentences, and tables of contents.
One user from Study 1 who scanned an article but failed to find what he was looking for said, "If this happened to me at work, where I get 70 emails and 50 voicemails a day, then that would be the end of it. If it doesn't come right out at me, I'm going to give up on it." "Give me bulleted items," another user said. While looking at a news site, one person said, "This is easy to read because it uses bold to highlight certain points." An essay containing long blocks of text prompted this response: "The whole way it looked made it kind of boring. It's intimidating. People want to read things that are broken up. It gets the points across better."
He's since revisited the subject of writing for the web many times -- highlighting keywords is a recurring theme. One of the first things you'll notice about Nielsen's own writing is he uses bold a lot.
I've been scolded for using too much bold in SO questions, so I learned that...
- Using too much bold disrupts your natural reading rhythm,
- but some italics and bold can be helpful in appropriate quantities.
- If your text contains a lot of highlighting or bold, consider breaking it up into bulleted lists.
I'm assuming you're referring to highlighting something in bold text only.
In this case my guideline is to use it, but don't count on it - it does help put the emphasis correctly for someone who actually reads the text (most users don't), but if you need to make sure the user pays attention then it's not enough.
Be sure not to highlight text with underline - it immediately implies a link.
In that regard, I tend to go for what government sites like direct.gov.uk do, and they don't bother, however they do break information up into clear and concise sections.
If you do it, I wouldn't go overboard. I think it does have its place in limited quantities to draw attention to important thoughts or points to be made. It certainly seems to work for a lot of internet sales landing pages. If you're looking for an example that doesn't look super spammy, take a look at some of the 37signals' product sales pages.
Remember search engines are looking for bold text. If you choose to do it - be SEO aware.
I remember there were many blogs and websites that used the yellow marker effect to highlight text but I don't see it that often anymore. I wonder why...