Does this tell me if the Deployment is valid Kubernetes?

No. It tells you whether the YAML parses. It does not apply the Kubernetes API schema.

Why can this page accept --- when the YAML converter cannot?

A converter has to produce one JSON or TOML value. This check only reports each document.

No. The check runs in your browser.

Checks each YAML document in a manifest. It does not check the Kubernetes schema.

A --- stream is checked one document at a time. apiVersion, required fields, and unknown kinds are not validated.

Ctrl+Enter or ⌘+Enter runs this tool immediately.

Manifest

Waiting for input.

Parsing stays in this browser. Input is not uploaded or saved.

Paste a manifest, or load the sample Deployment and Service.
Read the panel. An error names the document number and, when the parser has it, the line and column.
A successful check does not format the file or decide whether Kubernetes would accept it.

Each document is parsed as YAML 1.2 with the core schema. yes and no stay strings.
A stream of documents is allowed here. The single-document converters still reject ---.
Duplicate keys inside one document are errors.
This does not check apiVersion, kind, required fields, types, or unknown fields.
The manifest is not rewritten.

A Deployment and a Service separated by --- are two documents. Both must parse.

kind: Deployment
metadata:
  name: api
---
kind: Service
metadata:
  name: api

The error says Document 2 and the line. The first document can still be valid YAML.

kind: ConfigMap
metadata:
  name: one
---
kind: ConfigMap
metadata:
	name: two

Document 2 has a syntax error: Use the line and column. Fix that document without assuming the first one failed.
Duplicate key: Keep one spelling of the key inside that document.
Tabs used for indentation: Replace the tab with spaces. YAML does not accept a tab as indentation.

Does this tell me if the Deployment is valid Kubernetes?: No. It tells you whether the YAML parses. It does not apply the Kubernetes API schema.
Why can this page accept --- when the YAML converter cannot?: A converter has to produce one JSON or TOML value. This check only reports each document.
Is the manifest uploaded?: No. The check runs in your browser.

YAML validator
Check YAML syntax without converting or rewriting it. Errors include a line and column when the parser provides them, and valid input is left unchanged.
YAML to JSON converter
Convert YAML configuration to readable or compact JSON in the browser. Comments are dropped, and parse errors include line and column when the parser reports them.
YAML to TOML converter
Convert one YAML document to TOML in the browser. Comments are dropped, null is rejected, and multi-document streams are not merged.