Bad character “X” after “<”. Probable cause: Unescaped “<”. Try escaping it as “&lt;”.

The < character is the opening delimiter for HTML tags. When the browser’s HTML parser encounters <, it expects a valid tag name to follow (like div, p, span, etc.). If it instead finds an unexpected character—a digit, a symbol, a space, or anything that doesn’t form a recognized tag—the parser enters an error state. The validator flags this because the < was almost certainly meant as literal content, not as the start of a tag.

This issue commonly appears in several scenarios:

• Mathematical or comparison expressions like x < y or if (count < 10)
• Inline code or technical content that includes angle brackets, such as explaining generics (List<String>) or template syntax
• User-generated content or CMS output where text hasn’t been properly HTML-encoded
• Copy-pasting code snippets directly into HTML without escaping

Beyond triggering validation errors, unescaped < characters can cause real rendering problems. The browser may try to interpret the text following < as a tag, swallowing part of your visible content or producing unexpected DOM structures. This can also create accessibility issues, as screen readers rely on a well-formed DOM to convey content accurately.

How to fix it

Replace every < that appears as content (not as part of an actual HTML tag) with the HTML entity <. Similarly, replace literal > with >. While > doesn’t always cause validation errors, escaping both characters is a best practice that prevents ambiguity and keeps your markup clean.

If you’re writing code examples, wrap them in <code> or <pre> elements—but note that you still need to escape < and > inside those elements. The <pre> and <code> tags preserve whitespace and indicate code semantics, but they do not disable HTML parsing.

Examples

❌ Bad: unescaped < in a math expression

<p>Check if x < 10 before proceeding.</p>

The parser sees < 10 and tries to interpret 10 as a tag name, triggering the error.

✅ Fixed: escaped with <

<p>Check if x &lt; 10 before proceeding.</p>

❌ Bad: unescaped angle brackets in a code snippet

<pre><code>
List<String> names = new ArrayList<String>();
</code></pre>

The parser interprets <String> as an unknown HTML tag, breaking the content.

✅ Fixed: all angle brackets escaped inside <code>

<pre><code>
List&lt;String&gt; names = new ArrayList&lt;String&gt;();
</code></pre>

❌ Bad: unescaped < in a conditional expression

<p>The expression 1 < 2 && 3 > 1 evaluates to true.</p>

✅ Fixed: both < and > escaped

<p>The expression 1 &lt; 2 &amp;&amp; 3 &gt; 1 evaluates to true.</p>

Note that && also needs to be escaped as && because & is itself a reserved character in HTML.

Quick reference for common HTML entities

If you’re generating HTML dynamically (via JavaScript, a server-side language, or a CMS), make sure your output pipeline includes an HTML-encoding step. Most frameworks and template engines provide a built-in escaping function—use it for any user-supplied or variable content that gets inserted into HTML.

Más información:

MDN Reference

HTML text content and character references

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MDN Reference

HTML Character References

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WHATWG Spec

HTML syntax

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