Replaces the lines from 1 to the first occurrence of a line starting with STRING of every file found with find.

• find . -type f -name '*.html' returns a list of all files (not including directories) ending with .html. The ' ' in name is used to pass the literal wildcard * to the find command, instead of the * interpretation of bash, that is, repeat the command for every file in the current folder.
• -exec execute the following command in each of the files found, using {} as the filename. the ; termination must be escaped with \;
• sed -i replaces in file (output is the same file)
• -e '1r common_header' -e '1,/STRING/d' {} reads common_header file, then finds the first occurrence of STRING and replaces it, deleting all the previous lines and putting the contents of common_header.

The -i flag of sed requires a parameter in BSD sed, used as the suffix of a backup file. To not use a backup file, you can pass an empty value with -i''.