Highlight YAML is a browser-based syntax highlighter for YAML configuration and data files. Paste YAML into the editor and the preview immediately applies separate colors to mapping keys, quoted strings, numbers, booleans, null values, comments, anchors, aliases, directives and common YAML punctuation. The purpose is simple: make a dense YAML document easier to scan without opening a full code editor or sending the source to a server.
YAML is deliberately human-readable, but readability changes when a file grows beyond a small example. Deployment manifests, application settings, CI configuration, container definitions and automation files can contain many indentation levels, repeated list entries and mixed value types. When everything is rendered in one text color, it becomes harder to distinguish a key from its value, a comment from configuration data, or a number from a quoted string. This tool adds visual separation while keeping the original YAML text intact.
The input and preview are separate. The textarea contains the source exactly as you entered it, while the preview is generated from that source with escaped HTML and small syntax-specific span elements. Escaping matters because YAML can contain characters such as <, >, ampersands and quotes. The implementation treats those characters as source text instead of inserting raw user HTML into the page. The highlighting engine therefore does not need eval, an external code-highlighting library or an uploaded processing service.
The tool uses a practical pattern-based approach rather than pretending to be a complete YAML parser. Common block mappings are recognized so keys receive their own style. Scalar values are then classified where the syntax is clear: quoted text gets the string style, ordinary numbers receive the number style, common boolean spellings such as true, false, no, on and off are grouped together, and null or ~ is displayed as a null value. Full-line and trailing comments are separated visually. YAML document markers and directives also have dedicated styling. Common anchors and aliases are highlighted so references are easier to spot in larger configuration files.
Flow-style collections receive punctuation highlighting too. Brackets, braces and commas are shown differently from their surrounding values, which helps when a YAML file mixes normal indented blocks with compact lists such as ports: [3000, 8080]. The engine also understands common list markers, including mappings nested under a list entry. These rules are intentionally focused on visual clarity rather than semantic interpretation.
As the YAML changes, the statistics row updates at the same time. It reports total lines, detected keys, list items, comments and character count. These numbers are not a validity score; they are quick inspection metrics that help you confirm the text was pasted correctly and give you a rough sense of the document structure. An empty editor remains a valid state, and malformed or incomplete text can still be highlighted without a parsing request failing the whole widget.
The Load sample action makes the tool easy to test immediately. The sample includes nested mappings, lists, comments, quoted text, numbers, booleans, a null value and a flow-style array. The Clear action returns both the input and preview to an empty state. The live editor means there is no separate Highlight button to remember, so the main workflow stays focused on entering text and reading the result.
There are two output actions. Copy highlighted HTML copies the generated markup for the current YAML to the clipboard. This is useful when a highlighted snippet needs to be embedded into another page or documentation system that uses the same syntax classes. Download HTML creates a standalone HTML file containing the current highlighted YAML and the required styles. The exported file is useful for sharing a readable configuration snapshot without requiring the original tool page.
The tool is intentionally local and dependency-free. Highlighting, statistics, sample loading, copying and HTML generation happen in the browser. No external API is required by the implementation, and the widget does not need a server-side YAML processing endpoint. Very large YAML documents are still subject to the normal limits of a browser because the preview is regenerated from the text as it changes; unusually large files may take longer to render even though the source is not uploaded.
This distinction also matters for expectations. Highlight YAML is not a YAML validator. It does not attempt full YAML 1.1 or YAML 1.2 parsing, schema resolution, duplicate-key validation or semantic error reporting. It is appropriate when the task is visual inspection and presentation. When you need to prove that a configuration is syntactically valid, use a dedicated YAML validator after reviewing the highlighted result.
For everyday configuration work, the result is a compact workflow: paste, inspect, count, copy or export. The source remains untouched, the visual layer is generated safely, and the interface stays close to the rest of the tool collection instead of introducing an editor framework or a multi-step processing pipeline.