Expand additional data in the response. Use expand[]=field_name to include optional fields (e.g., expand[]=head_code&expand[]=footer_code&expand[]=custom_css&expand[]=markup). Markup is opt-in on every endpoint (show, list, create, update) because PML serialization runs per page. custom_css is available on internal pages only. See the Expanding guide for available fields and examples. Read a page's existing markup with expand[]=markup before editing it: see Reading Current Markup in the Pages Skill.