About This HTML Issue
The poster attribute specifies an image to display as a placeholder while the video is loading or before the user starts playback. Like all HTML attributes that accept URLs — such as src, href, and action — the value must conform to valid URI syntax as defined by RFC 3986. In this standard, a literal space character is not a legal character in any part of a URL. When the validator encounters a space in the poster attribute’s value, it flags it as an illegal character in the path segment.
While most modern browsers are forgiving and will attempt to resolve URLs containing raw spaces by internally encoding them, relying on this behavior is problematic for several reasons:
- Standards compliance: The HTML specification requires valid URLs. Raw spaces violate this requirement.
- Interoperability: Not all user agents, HTTP clients, or content delivery systems handle unencoded spaces the same way. Some may truncate the URL at the first space or fail to resolve the resource entirely.
- Portability: If your HTML is consumed by tools, scrapers, or APIs that strictly parse URLs, unencoded spaces can cause silent failures.
- Consistency: Keeping URLs properly encoded prevents subtle bugs when paths are constructed dynamically in server-side or client-side code.
The fix is straightforward. You have two options:
-
Percent-encode the spaces: Replace every space in the URL with
%20. This preserves the original file and folder names on the server while producing a valid URL in your HTML. -
Eliminate spaces from file and folder names: Use hyphens (
-), underscores (_), or camelCase instead of spaces. This is generally considered best practice for web assets, as it avoids encoding issues across the board.
Note that this rule applies to the entire URL path, not just the filename. If any directory in the path contains a space, it must also be encoded or renamed. The same principle applies to other special characters that are reserved or disallowed in URLs, such as {, }, |, ^, and [.
Examples
Incorrect — space in the path
The folder name video images contains a space, which is illegal in a URL path segment.
<video controls poster="/img/video images/snapshot.png">
<source src="/videos/sample.mp4" type="video/mp4">
</video>Incorrect — space in the filename
The filename my poster.jpg also triggers the same error.
<video controls poster="/img/my poster.jpg">
<source src="/videos/sample.mp4" type="video/mp4">
</video>Fixed — percent-encoding the spaces
Each space is replaced with %20, producing a valid URL.
<video controls poster="/img/video%20images/snapshot.png">
<source src="/videos/sample.mp4" type="video/mp4">
</video>Fixed — removing spaces from the path
Renaming the folder to use a hyphen eliminates the need for encoding entirely.
<video controls poster="/img/video-images/snapshot.png">
<source src="/videos/sample.mp4" type="video/mp4">
</video>Fixed — removing spaces from the filename
<video controls poster="/img/my-poster.jpg">
<source src="/videos/sample.mp4" type="video/mp4">
</video>
As a general best practice, avoid spaces in all file and folder names used on the web. Use hyphens or underscores instead. If you’re working with files you can’t rename — such as assets from a CMS or third-party system — always percent-encode spaces as %20 in your HTML. This applies not only to poster but to every attribute that takes a URL value, including src, href, action, data, and formaction.
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