I have two dataframes df1 and df2. I would like to highlight in yellow all the rows in df1 that are also present in df2.

Alternatively, to actually do the comparison inside the function, define a set of tuples of df2 rows beforehand:

set_df2 = set(df2.apply(tuple, axis=1))

def highlight_rows(row):
    color = 'yellow' if tuple(row) in set_df2 else ""
    return [f'background-color: {color}'] * len(row)

df1.style.apply(highlight_rows, axis=1)