Find the button, attach a listener, change something on the page when it's clicked. Three steps, and two mistakes that make it silently do nothing.
Your HTML file opens blank, shows as text, or downloads instead of rendering. Check the extension, the file:// open path, the doctype and a missing closing tag before you rewrite the page.
Measure two boxes with the same declared width, calculate padding and border, and reproduce the 100% width overflow mistake before fixing it.
Compare text alignment, a centred block and two-axis child placement, then test why centering needs available space.
Diagnose an ID rule that defeats a reusable note modifier, then keep the ID for navigation while moving presentation to classes.
Set a shared typography base, trace inherited colours and remove a local override without restyling every paragraph.
Trace a reproducible colour and spacing conflict from the rendered result to the winning rule, then verify the repair in the source file.
Build a local form with working labels and a grouped topic choice, then inspect submitted field names without sending a message.
Choose different text alternatives for informative, decorative and linked images, then inspect their names and broken-image behavior.
Build a three-file local site, resolve paths from nested folders and test fragments and deliberate new-tab navigation.
Build a page with named navigation regions and a working skip link, and distinguish page landmarks from an article header.