Vivaldi has Reader Mode feature with version 1.5, and user can assign any keyboard shortcut to it and use whenever they find articles with long text in browser. The Vivaldi team has now defined a new default keyboard shortcut for Reader View, which is Alt+R. You can change this and create another one to your liking by visiting the ‘Page’ section in the Keyboard Shortcuts of Settings.
ICYMI recently got the feature a configuration panel, “which allows you to change the font type, font size, column width, row height and page style to light, dark or theme mode”.
Vivaldi 1.11 adds Configuration Panel for Reader Mode
For any keyboard shortcut, including this one, you can restore the default shortcut, if modified, or delete the existing one. The latest version of snapshot 1.11 has this change, it will be publicly available when Vivaldi ships the stable version.
To modify or view the Reader View shortcut, click on Vivaldi menu > Settings > Keyboard, expand the Page section and check the ‘Toggle Reader View’ field.
And the Firefox Reader View keyboard shortcut is
Firefox 53 shows reading time in reader mode
Firefox 54 shows Reader View shortcut in View menu
Firefox Simplified Page Keyboard Shortcut