Showing HTML of two plots generated with to_html fails

[augustebaum]

What happened?

See screenshots, along with the following snippet to reproduce.
I have a slight suspicion this is related to #3435.

With to_html:

Without to_html:

import altair as alt
import numpy as np
import pandas as pd
from altair.utils._show import open_html_in_browser

A = np.array([[50, 0, 0], [0, 48, 2], [0, 2, 48]])

data = pd.DataFrame(
    ((i, j, v) for (i, j), v in np.ndenumerate(A)), columns=["i", "j", "v"]
)
chart = (
    alt.Chart(data, title="Confusion matrix")
    .mark_rect()
    .encode(
        x=alt.X("i:O", axis=alt.Axis(orient="top"), title="Predicted class"),
        y=alt.Y("j:O", title="True class"),
        color="v:Q",
    )
    .properties(width=400, height=400)
)

# only shows one plot
open_html_in_browser(f"{chart.to_html()} {chart.to_html()}")


def altair_to_html(chart):
    from altair.vegalite.v5.display import html_renderer

    return html_renderer(chart.to_dict())["text/html"]


# correctly shows 2 plots
open_html_in_browser(f"{altair_to_html(chart)} {altair_to_html(chart)}")

What would you like to happen instead?

Both methods should have the same behavior.

Which version of Altair are you using?

5.3.0