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This post is auto-generated from RSS feed The Rust Programming Language Forum - Latest topics. Source: Strict HTML validation in Rust: How to detect unclosed tags and unresolved placeholders?

Overview

I'm working on a Rust project that involves validating HTML content—both full documents and fragments. My requirements go beyond lenient parsing:

• Detect and flag unclosed or extra HTML tags
• Identify unresolved template placeholders such as {{field}}, {{ user.name }}, or malformed ones like {{{bad}}}, {{ incomplete
• Avoid relying on parsers that silently fix malformed HTML (like html5ever)

Problem

Crates like html5ever and markup5ever_rcdom follow the forgiving HTML5 spec and parse even broken HTML successfully. This makes it impossible to detect structural issues at runtime.

For example, this invalid HTML:

<div class="row template-row">
  <div class="col-auto">
    <a> <!-- ❌ Unclosed <a> tag -->
  </div>
</div>

The above is invalid HTML, yet parsers like html5ever auto-close the <a> tag, and the document is considered "valid" at the parsing level.

This becomes a problem in scenarios like Angular templates, where unclosed tags can cause runtime or build-time errors. Structural correctness matters more than "valid per HTML5 spec."

What I'm Looking For

I'm searching for a Rust-based approach that supports the following validation features:

1. Structural Validation

• Detect unclosed, mismatched, or stray tags
• Ex: <div><a></div> or <div><span> should throw an error

2. Placeholder Detection

• Find unresolved or malformed template variables like {{field}}, {{, {{{bad}}}

3. Fail-Fast Parsing

• Prefer errors over silent recovery
• Option to opt out of "forgiving mode" if using an HTML5 parser

Questions

• Is there any existing Rust crate that performs strict HTML validation (not just parsing)?
• Can we configure html5ever to operate in a strict mode or report unclosed tags?
• Is writing a custom tag-matching validator (like a stack-based parser) the only way?
• Would wrapping an external tool like tidy from Rust be the most pragmatic solution?
• Are there template-aware linting crates for formats like Angular, Handlebars, etc. in Rust?

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