What is parser-blocking?

This term is related to parsing of a string of HTML code by a browser.

Parser-blocking generally refers to scripts that are downloaded and executed when the HTML parser comes across a <script> tag. When the parser comes across a <script> tag, it goes ahead and downloads the script. And as soon as it is available in the browser, the default behavior is that the browser goes on to execute the script right after download.

Both download and execution of the script halts the HTML parser. So the parser has to wait till both processes are complete before it can continue to parse the next character in the string. This is called parse-blocking.

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References

1. Parse
2. How the browser renders a web page? — DOM, CSSOM, and Rendering