Here's a list of some unexpected behaviors of the HTML repr of meta-estimators

• Pipeline doesn't display its named steps

  

  This was maybe intentional ? In comparison FeatureUnion does for instance

  

• a Pipeline in a meta-estimator doesn't render properly; it doesn't have the dashed border

  

  Another meta-estimator renders properly

  

• transformers of ColumnTransformer are expandable to show the selected columns, but there's no additional info. I think it should be explicit that these are the selected columns.

  

  Note that this could be fixed by ENH: Display the number of output features #31937

• When the inner estimator of a meta-estimator is not a meta-estimator itself, it's expandable but the dropdown is not useful anymore (it's the non-html repr of the estimator basically):

  

  Now that we have the parameter table, it's not useful anymore to have the additional repr which is redundant and less informative. I'd be in favor of removing the dropdown

• When the inner estimator of a meta-estimator is a meta-estimator itself, it's expandable but sometimes there's no dropdown or the dropdown is the non-html repr of the meta-estimator, and there's no parameter table.

  

  I think that it should display the parameter table instead in both cases