A highlight is a reaction post of a range of content that consists typically of just a restyling (like a yellow background as you might use a yellow physical highlighter, or an underline).
People highlight books and other physical texts, as well as do so with revisions in proprietary software like Microsoft Word, thus there is some evidence they may want to do so with posts on the web as well.
No specific how to post a highlight post has been determined. More real world examples are needed for study before reasoned proposals can be made / implemented that would be able to support a desired presentation.
Chris Aldrich has been experimenting with highlighting in posts since around October 2016. In some read posts, in which he also archives the original post, he uses the html <mark> tag sometimes with a class using his initials and/or a date/time stamp (when the post date/time don't otherwise agree with the original highlight time) to indicate the originator and date/time of the highlight. The color of the highlight can be easily handled in CSS and as the mark html tag is not often used, it doesn't typically conflict with the uses via other sites.
Because of the way his read posts are created, the matching of the original canonical URL allows other highlighting/annotation layers, like Hypothes.is for example, to also add their highlights and annotations over his pages as well. See note at Pressforward and Hypothesis Work Great Together. In some sense this gives him the ability to recreate the functionality of turning on/off the highlights of others, a functionality offered by Amazon Kindle (see section on https://www.amazon.com/gp/help/customer/display.html?nodeId=201241990 View Popular Highlights & Public Notes].
He's considering using a microformat (perhaps `p-highlight-of`) on highlights for parsers to be able to find/consume them in the future to differentiate between other similar mark up including `u-quotation-of` and `h-cite`.
In book reviews since Summer 2016, he has also been adding blockquotes containing highlighted text exported from his Amazon Kindle indicating a particular highlighter color which he also uses to delineate particular meanings. Example: Guide to Highlight Colors.
Another older example of a highlight on his site is Continuum functional integrals. This was created as a quotation at the time (given the technology), but the original intent was to highlight the particular quote with a much longer textbook. Instead of including simply a quotation, he included a photograph of the portion of the text in the post along with a blockquoted copy he meant to highlight.
Kartik Prabhu uses a version of bookmark + fragmention to effectuate highlights on his site. Example: https://kartikprabhu.com/notes/2016-12#bm-resilientweb-choose-tools-wisely
Hypothes.is allows one to both highlight and add notes/marginalia onto an overlapping layer on pages. These notes have permalink URLs and they can also be shared to other sites.
Medium may have features resembling highlighting.
Kevin Marks has highlighted text within Twitter using photos of actual texts. This relatively frequently used functionality (though often using screenshots) is commonly used to get around the 140 character limit found on Twitter.
Other Examples of Photos of Highlighted Text
Needs more research, documentation, screenshots.
bookmark a fragmention
The receiver of the highlight reaction, could determine this automatically, and decide to provide highlighting with a default highlight color that goes well with their post styling.
Alternatively, since highlights of paper text typical consist of the person doing the highlighting making a deliberate choice of how to stylistically express the highlighting, that stylistic expression could be considered as an additional attribute / property of a highlight post.