About This HTML Issue
The handheld media type is deprecated, and the validator rejects any media attribute on a <link> element that still lists it.
CSS 2.1 defined handheld for small-screen devices with limited bandwidth. Few browsers honored it. Safari on the iPhone ignored it and applied screen styles, and so did the smartphone browsers that came after. Media queries based on viewport width took over as the way to adapt a layout to small screens. Media Queries Level 4 deprecated handheld together with tty, tv, projection, braille, embossed and aural. Browsers still accept these types but never match them, so a stylesheet linked with media="handheld" is never applied.
The validator quotes the whole attribute value, and it reports the error even when handheld sits in a list next to a valid type, as in media="screen, handheld". In that case, delete handheld and keep the rest. If the stylesheet was meant for phones, replace handheld with a media query such as (max-width: 480px). If it should apply everywhere, remove the media attribute.
Modern browsers have been skipping any stylesheet marked handheld, so switching it to a media query makes its rules apply for the first time. Review those rules before you make the change.
Invalid example
<link rel="stylesheet" href="main.css">
<link rel="stylesheet" href="mobile.css" media="handheld">
Valid example
<link rel="stylesheet" href="main.css">
<link rel="stylesheet" href="mobile.css" media="(max-width: 480px)">
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