If you google “Double quotes or single quotes inside html tag, which is better”, you might hear someone is talking about using double quotes in html won’t pass XHMTL validation. I think it is nonsense.
Both of these characters are valid for XHTML, i.e. both
<tag att="value">
and
<tag att='value' >
are valid. However, I have a good reason to encourage you to stick to double quotes.
When value is taken from user input, it is a must to convert all applicable characters to HTML entities by htmlentities(), and htmlentities() default flag is ENT_COMPAT which convert double quotes but leave single quotes alone. That is to say,
If I use
<tag att="(value taken from user input)">
constantly in templates, and if htmlentities() converts all double quotes in user input, whatever user input is, it will not break html output.
I used to use double quotes and single quotes interchangeably, but I learned from Magento. Magento sticks to using double quotes. Its htmlEscape() bridges to htmlspecialchars() but it does not let you pass in a flag to control how to convert double quotes or single quotes. So, if you use
<tag att='(value taken from user input)'>
, and if you rely on Magento htmlEscape() to convert special charcters for you, when user input has some single quotes, it ruins html output. I am sure you do not want it happens, so stick to using double quotes, always.