Lazy continuation or strict marker requirement for indented lines in Markdown block quotes

Resolution

To ensure consistent rendering across all compliant parsers, every line within a block quote must begin with the > marker (optionally preceded by up to three spaces). Parsers should not treat a line starting with three spaces and no marker as a lazy continuation of the block quote; instead, such a line should be evaluated as a potential indented code block or a new paragraph outside the quote.

Technical Explanation

The conflict arises from a legacy behavior known as "lazy continuation," where some early Markdown implementations allowed lines without a > to remain part of a block quote if they were indented. However, modern specifications—specifically CommonMark—have moved toward strict marker requirements to eliminate ambiguity.

The Ambiguity of Indentation

When a line starts with three or four spaces, it triggers a conflict between two different Markdown rules:

• Block Quote Continuation: The assumption that the indentation is simply for visual alignment within the quote.
• Indented Code Blocks: The rule that any line indented by four spaces (or sometimes three, depending on the parser) is a preformatted code block.

If a parser treats an indented line as a lazy continuation, it renders as standard text within the <blockquote>. If it treats it as a code block, it renders as a <pre> block, potentially breaking the quote's structure or ending the quote entirely.

Implementation and Compatibility Strategy

For implementers and authors seeking maximum compatibility, follow these guidelines:

1. Avoid Lazy Continuation: Do not rely on indentation to keep text inside a block quote. Always use the > marker.
2. Prioritize Specification: Follow the CommonMark standard, which mandates the marker. This prevents the parser from guessing whether the user intended a code block or a quote continuation.
3. Explicit Fencing: If a code block is intended inside a quote, use fenced code blocks ( ``` ) preceded by the > marker on every line.

Verification Method

To verify how a specific parser handles this edge case, test the following snippet:

> This is a quote
   This line has three spaces and no marker

• Strict/CommonMark: The second line will be outside the <blockquote>.
• Lazy Parser: The second line will be inside the <blockquote>.

Diagnostic Detail: To provide a more specific recommendation, please identify the specific Markdown flavor (e.g., GFM, CommonMark, MultiMarkdown) or the specific library (e.g., markdown-it, Kramdown) being used.