What is Best syntax highlighter for Joomla?
Geshi is not working fine in my site.
How to integrate this style of syntax in Joomla 3 site?
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There are a few third party extensions in the Joomla Extensions Directory that can enable code highlighting:
Easy Code Highlighter (paid)
Script Code Highlighter (free)
JExtBOX Code Display (paid)
"Best" is a little subjective given that you haven't specified any requirements or preferences. I recommend downloading and testing the free ones to see which of these most closely fits your requirements. If none of the free options are sufficient, then you may need to investigate the paid options.
Codemirror is included in joomla.
You can use the formfield "editor" and select codemirror with editor="Codemirror". Take a look at https://docs.joomla.org/Editor_form_field_type