About This HTML Issue
The <strike> element is no longer part of HTML, so replace it with <del> for text removed from the document or <s> for text that is no longer accurate.
<strike> only told the browser to draw a line through some text. It said nothing about why the line was there, so HTML5 dropped it along with other presentational elements such as <big>, <center> and <font>. Browsers still render it, but the validator reports every use as an error.
The two replacements mean different things. <del> marks content that was deleted from the document, such as a corrected time in an announcement. It accepts cite and datetime attributes to record the source and the date of the change, and it often sits next to an <ins> element holding the new text. <s> marks content that is still shown for context but is no longer accurate or relevant, such as the old price next to a sale price. When the line is only decoration, use a <span> with the CSS text-decoration: line-through instead.
Rich text editors and old CMS templates still produce <strike>, sometimes as empty <strike></strike> pairs left over from formatting. Change the editor's strikethrough output if it has a setting for it, or clean up the saved HTML. Empty pairs can simply be deleted.
Invalid example
<p>Ticket price: <strike>$49</strike> $29</p>
<p>The meeting starts at <strike>10:00</strike> 11:00.</p>
Valid example
<p>Ticket price: <s>$49</s> $29</p>
<p>The meeting starts at <del>10:00</del> <ins>11:00</ins>.</p>
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