Evidence regarding the use of highlighting for benefit in learning is mixed per Carole L. Yue et.al. in 2014 and under some conditions, highlighting can be a beneficial study strategy for
learning. Quoting Andy DeSoto's answer in 2012, "it might make sense to only use highlighting (or colors, fonts, etc.) to highlight key points, etc.; much like what we already do in writing for the web. Like anyone who highlights a textbook knows, it doesn't do any good to highlight an entire chapter."
Potential Advantages of Highlighting
Layout/color can be used to improve text readability per studies mentioned below.
The act of deciding what to mark and what not to mark may lead students to process textual information at a deeper, more evaluative level than they would when simply reading it per researchers Craik/Lockhart 1972 and Nist/Hogrebe 1987.
Learner generated highlighting tends to produce better test performance than experimenter-generated highlighting per researchers Fowler and Barker 1974, Rickards and August 1975, Rickards and Denner 1979, Nist and Hogrebe 1987.
When students read pre-highlighted passages, they recall more of the highlighted information and less of the non-highlighted information as compared to students who receive an unmarked copy of the same passage per researchers Fowler and Barker 1974 and Silvers and Kreiner 1997.
When students are trained in highlighting techniques (i.e., to read a paragraph, decide what is conceptually important, and then highlight that information), they perform better than students who do not receive such training per research by Leutner et al. 2007.
Potential Disadvantages of Highlighting
Selectively highlighting text might be ineffective or even detrimental to learning per Dunlosky et al. 2013, Idstein and Jenkins 1972, Stordahl and Christensen 1956. One argument is that students often do not know how to highlight effectively, so such activity primarily amounts to a mechanism for tracking progress and does not involve deeper processing per Stordahl and Christensen, 1956 and Bell and Limber, 2010.
Forcing readers who never use highlighters to do so may interfere with their learning and prevent them from employing the type of encoding techniques they usually find beneficial per Howe and Singer, 1975 and Brown and Smiley 1978.
Highlighting through underlining could ironically impair memory for critical information by preventing students from restudying the information in a way that effectively promotes long-term retention per Peterson 1992.
Research by Peterson in 1992 had education majors read a 10,000-word chapter from a history textbook; two groups underlined while studying for 90 minutes, whereas a third group was allowed only to read the chapter. One week later, all groups were permitted to review the material for 15 minutes prior to taking a test on it (the two underlining groups differed in whether they reviewed a clean copy of the original text or one containing their underlining). Everyone received the same test again 2 months later, without having another chance to review the text. The multiple-choice test consisted of 20 items that probed facts (and could be linked to specific references in the text) and 20 items that required inferences (which would have to be based on connections across the text and could not be linked to specific, underlined information).
The three groups performed similarly on the factual questions, but students who had underlined (and reviewed their marked texts) were at a disadvantage on the inference questions. This pattern of results requires replication and extension, but one possible explanation for it is that standard underlining draws attention more to individual concepts (supporting memory for facts) than to connections across concepts (as required by the inference questions).
Per Dunlosky's overall assessment of highlighting in 2013, "On the basis of the available evidence, we rate highlighting and underlining as having low utility. In most situations that have been examined and with most participants, highlighting does little to boost performance. It may help when students have the knowledge needed to highlight more effectively, or when texts are difficult, but it may actually hurt performance on higher level tasks that require inference making. Future research should be aimed at teaching students how to highlight effectively, given that students are likely to continue to use this popular technique despite its relative ineffectiveness."