Use the spaceless tag to remove whitespace between HTML tags, not whitespace within HTML tags or whitespace in plain text:
{% spaceless %}
<div>
<strong>foo</strong>
</div>
{% endspaceless %}
{# output will be <div><strong>foo</strong></div> #}
This tag is not meant to "optimize" the size of the generated HTML content but merely to avoid extra whitespace between HTML tags to avoid browser rendering quirks under some circumstances.
Tip
If you want to optimize the size of the generated HTML content, gzip compress the output instead.
Tip
If you want to create a tag that actually removes all extra whitespace in an HTML string, be warned that this is not as easy as it seems to be (think of textarea or pre tags for instance). Using a third-party library like Tidy is probably a better idea.
Tip
For more information on whitespace control, read the dedicated section of the documentation and learn how you can also use the whitespace control modifier on your tags.
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https://twig.sensiolabs.org/doc/1.x/tags/spaceless.html