Cross-site scripting, or XSS, is a security vulnerability that can be found in web applications. This vulnerability allows attackers to inject malicious code/styles into a web page viewed by users. Hackers trying to attack in HTML code to attack / harm files
PHTML templates
An ‘Escaper’ class is provided for .phtml templates and PHP classes responsible for generating HTML. It contains HTML sanitization methods for a variety of contexts.The following code sample illustrates XSS-safe output in templates:
<?php echo $block->getTitleHtml() ?> <?php echo $block->getHtmlTitle() ?> <?php echo $block->escapeHtml($block->getTitle()) ?> <?php echo (int)$block->getId() ?> <?php echo count($var); ?> <?php echo 'some text' ?> <?php echo "some text" ?> <a href="<?php echo $block->escapeUrl($block->getUrl()) ?>"><?php echo $block->getAnchorTextHtml() ?></a>
The $block local variable available inside .phtml templates duplicates these methods.
Follow below adobe commerce link to know more details
https://devdocs.magento.com/guides/v2.3/extension-dev-guide/xss-protection.html