Read-mode eliminates any unnecessary items on the web URL reference, such as headers, footers and advertisements to help users focus on the information.
Enable read-mode by pressing the button on the top right of the split-viewer when viewing URL references.
You can highlight information on read-mode by actually highlighting the targeted information and clicking on the Highlighter icon that pops up.
Highlights are saved in the Document Library
Highlights are saved individually but clumped under the reference where it was highlighted from.