Is it ok to break long html elements down into multiple lines? I often see very long code lines like this:

&lt;input type="email" name="email" autocomplete="email" id="email_address" value="&lt;?= $block-&gt;escapeHtmlAttr($block-&gt;getFormData()-&gt;getEmail()) ?&gt;" title="&lt;?= $block-&gt;escapeHtmlAttr(__('Email')) ?&gt;" class="input-text" data-mage-init='{"mage/trim-input":{}}' data-validate="{required:true, 'validate-email':true}"&gt;


Is it ok to break them down like:

&lt;input type="email" 
       name="email" 
       autocomplete="email" 
       id="email_address" 
       value="&lt;?= $block-&gt;escapeHtmlAttr($block-&gt;getFormData()-&gt;getEmail()) ?&gt;" 
       title="&lt;?= $block-&gt;escapeHtmlAttr(__('Email')) ?&gt;" 
       class="input-text" 
       data-mage-init='{"mage/trim-input":{}}' 
       data-validate="{required:true, 'validate-email':true}"
&gt;


Because this makes it much clearer and easier to read in my opinion.

I usually do this if there are more than about 80-100 chars.



Validation with https://validator.w3.org/
I'd already answered this in the comments (which I shouldn't, we got answer fields for this), but anyway...

Because this makes it much clearer and easier to read in my opinion.

Absolutely. And that's a good reason to do it. Your 80-100 character limit makes sense as well. 80 is considered a standard 'limit' of sorts in many languages (Python is famous for it, but back in the day punch cards and teletypes were already limited to 80 columns).
Google's HTML style guide states:

Break long lines (optional).
While there is no column limit recommendation for HTML, you may consider wrapping long lines if it significantly improves readability.
When line-wrapping, each continuation line should be indented at least 4 additional spaces from the original line.

You've already verified your validator doesn't have a problem with it either, so, go for it. In hindsight, if the validator would've had problems with it, it would've been time for a better validator.
Better readability =&gt; better maintainability =&gt; less bugs and less development time.