HTML can, of course, be checked using any number of HTML-aware editors and linters. However, formatting systems typically provide extension properties and functions that aren’t supported by browsers. Some of these ‘extensions’ are properties originally defined in CSS Working Drafts but dropped from later versions.

Extensions are, by definition, not universally supported, and the same is true for their editing and validation. The CSS editor in Oxygen, for example, recognises the extensions for Oxygen Chemistry, but not the extensions for other formatters. An Oxygen framework cannot extend the syntax checking of the Oxygen CSS editor, so Antenna House provides a VSCode extension that recognises the Antenna House CSS Formatter extension properties (but not the Oxygen extensions).

The VSCode extension supports syntax highlighting of the Antenna House extensions and provides tooltips that show a property’s definition and allowed values plus links to the property’s documentation and code samples.