Add debug mode - Debug mode is enabled with a URL flag `debug=true`. - When debug mode is enabled, `visibility: hidden` is not set to the page during layout.
Add debug mode
- Debug mode is enabled with a URL flag `debug=true`.
- When debug mode is enabled, `visibility: hidden` is not set to the page during layout.

vivliostyle/vivliostyle-ui,vivliostyle/vivliostyle.js,vivliostyle/vivliostyle-ui,vivliostyle/vivliostyle-ui,vivliostyle/vivliostyle.js,vivliostyle/vivliostyle-ui,vivliostyle/vivliostyle.js,vivliostyle/vivliostyle.js